Abstract
Objective:
To investigate the hypothesis that folic acid supplementation and dietary folate intake before conception and during pregnancy reduce the risk of small for gestational age (SGA) and to examine the joint effect of folic acid supplementation and dietary folate intake on the risk of SGA.
Design:
Participants were interviewed by trained study interviewers using a standardized and structured questionnaire. Information on birth outcomes and maternal complications was abstracted from medical records and dietary information was collected via a semi-quantitative FFQ before conception and during pregnancy.
Setting:
A birth cohort data analysis using the 2010–2012 Gansu Provincial Maternity and Child Care Hospital.
Participants:
Women (n 8758) and their children enrolled in the study.
Results:
Folic acid supplementation was associated with a reduced risk of SGA (OR = 0·72, 95 % CI 0·60, 0·86), with the reduced risk seen mainly for SGA at ≥37 weeks of gestational age (OR = 0·70, 95 % CI 0·58, 0·85) and nulliparous SGA (OR = 0·67, 95 % CI 0·54, 0·84). There was no significant association between dietary folate intake and SGA risk.
Conclusions:
Our study suggested that folic acid supplementation was associated with a reduced risk of SGA and the risk varied by preterm status and parity.
Keywords: Folic acid supplementation, Dietary folate intake, Small for gestational age, Cohort study
Small for gestational age (SGA) increases neonatal mortality and various infant morbidities, including chronic lung disease, necrotizing enterocolitis, perinatal acidosis, hypoglycaemia, hypothermia, coagulation abnormalities and selected immunological deficiencies(1,2). It also leads to chronic diseases in later life such as type 2 diabetes, hypertension, obesity, CVD and mental health problems(3–9). The associated economic costs due to immediate neonatal intensive care, ongoing long-term complex health needs and lost economic productivity can be substantial. Unfortunately, modern obstetrics is still unable to predict, prevent or treat SGA(10).
Folate acts as a coenzyme in the biosynthesis of purine nucleotides and deoxythymidylic acid which are essential for DNA and RNA synthesis(11). While folic acid supplementation has been recommended for prevention of neural tube defects, its impact on other birth outcomes is not fully understood. Epidemiological studies investigating the associations of folic acid supplementation and dietary folate intake with SGA have provided conflicting results(12–34). Eleven studies found that folic acid supplementation before and/or during pregnancy reduced the risk of SGA(15–17,20,22,26,28,30,32–34), but three studies reported no association between folic acid supplementation and SGA(14,25,31). In addition, three studies suggested that a high dosage of folic acid may be associated with an increased risk of SGA at birth(23,24,27). Two studies found that dietary folate intake during periconception or pregnancy was a protective factor for SGA(17,27), but one study found that SGA was not associated with dietary folate intake during the second trimester(25). Five studies reported that higher folate concentrations in blood (erythrocyte or serum) during preconception or pregnancy had a protective effect on SGA(12,13,18,19,21), but one study found that SGA was not associated with folate concentrations(29). Today, folic acid supplementation and dietary folate intake are recommended to women in many countries. A recommendation to take folic acid supplements starting from 3 months before pregnancy until the end of the first trimester of pregnancy to prevent neural tube defects has been exercised in China since 2009 and users take 400 µg folic acid daily(35). National health authorities in many countries recommend periconceptional folic acid supplementation, and some countries have introduced mandatory folate fortification of foods(36–40). By taking account of the joint effect of folic acid supplementation and dietary folate intake, it might be possible to define the folic acid supplementation scheme mostly likely to affect SGA risk. However, no study has examined the joint effect of folic acid supplementation and dietary folate intake on the risk of SGA.
Approximately one-fifth of the world’s population is Chinese. The rate of SGA infants has increased over recent years, with the reported rate of SGA ranging from 5·82 to 17·5 %(22,31,33,34). The typical diet of northern China is characterized by low amounts of fresh vegetables and fruits, particularly during the winter, resulting in low blood concentrations of folate(41). Only 10–15 % of women of childbearing age routinely take folic acid supplements(42–44). Given the low percentage of folic acid supplementation among women of childbearing age, seasonal variation of dietary folate intake and the lack of folic acid fortification in staple foods in China, the Lanzhou birth cohort study provides a unique opportunity to concurrently study the impact of folic acid supplementation and dietary folate intake on SGA.
Materials and methods
Study population
A birth cohort was conducted in 2010–2012 at the Gansu Provincial Maternity and Child Care Hospital (GPMCCH), the largest maternity and child care hospital in Lanzhou, China(45–48). After obtaining written consent, an in-person interview was conducted at the hospital by trained study interviewers using a standardized and structured questionnaire to collect information. Maternal characteristics (age, income, education level, parity, BMI, etc.), social factors (employment status, height of partner, etc.), lifestyle factors (smoking status, drinking, etc.) and pre-existing medical and previous obstetric history (history of preterm birth, caesarean section, etc.) were self-reported. Pregnancy complications and birth outcomes (pre-eclampsia, gestational diabetes, birth weight, gender of live birth, etc.) were based on diagnoses from the medical records. All women were interviewed within 1–3 d after delivery. A total of 14 359 eligible women were approached for participation and 10 542 (73·4 %) women completed in-person interviews, with 10 179 singleton live births. Among 363 fetuses, 323 were multiple fetuses and forty fetuses died in utero (>32 weeks) or were stillborn. All study procedures were approved by the human investigation committees at the GPMCCH and Yale University.
Gestational age at delivery was calculated in completed weeks from the first day of the last menstrual period. All self-reported last menstrual period dates were further verified by ultrasound examinations during antenatal care in the hospital. SGA was defined as a birth weight below the 10th percentile of the gestational-age- and gender-specific birth weight standard for Chinese newborns(49); large for gestational age was defined as a birth weight greater than the 90th percentile of the standard; while appropriate for gestational age (AGA) was defined as a birth weight between the 10th and 90th percentile of the standard. The range of gestational age in the Chinese national standard was from 28 to 44 weeks(49). For neonates with a gestational age of 22–27 weeks, the US national reference based on 2009–2010 live births was applied as a surrogate(50). After exclusion of large-for-gestational-age births, 8758 (784 SGA and 7974 AGA) were included in the final analysis.
Folic acid supplementation and dietary folate intake
Data collection on folic acid supplementation and dietary folate intake has been described in a previous study and the FFQ was validated(47). Folic acid supplementation started from 3 months before pregnancy through to the end of the first trimester of pregnancy and users took 400 µg folic acid daily. Briefly, information on folic acid supplements was asked for the following four time periods: preconception (12 months before pregnancy), first trimester (1–13 weeks), second trimester (14–27 weeks) and third trimester (>27 weeks). For each time period, the duration and frequency of folic acid supplementation alone and of folic acid-containing multivitamins were ascertained. Folic acid supplementation users were defined as those who took folic acid supplements alone or folic acid-containing multivitamins before conception or during pregnancy. Preconception and pregnancy users were defined as those who took folic acid supplements alone or folic acid-containing multivitamins before conception and during pregnancy. Preconception-only users were defined as those who took folic acid supplements alone or folic acid-containing multivitamins before conception only. Pregnancy-only users were defined as those who took folic acid supplements alone or folic acid-containing multivitamins during pregnancy only. Non-users were defined as those who never took folic acid supplements alone or folic acid-containing multivitamins before conception and/or during pregnancy. The final variables included folic acid supplement users and non-users; folic acid supplements were classified into three levels by duration of use: before conception and during pregnancy, before conception only and during pregnancy only.
Dietary information was collected via a semi-quantitative FFQ. Daily dietary folate intake was estimated from the frequency of consumption and portion size of food items using the Chinese Standard Tables of Food Consumption(51).
Statistical analysis
Pearson’s χ 2 tests were used to compare selected characteristics between AGA and SGA. Unconditional logistic regression models were used to estimate the odds ratio and 95 % confidence interval for single associations of folic acid supplementation and dietary folate intake with SGA. The interaction analysis of multiplication models, done by logistic regression modelling, estimated the joint effect of folic acid supplementation and dietary folate intake on SGA. Dose–response relationships (and P trend) were calculated by including those categorical levels. According to the literature and the results of univariate analysis, potential confounders including maternal age, monthly income per capita, maternal education level, smoking, maternal employment, pre-pregnancy BMI, weight gain during pregnancy, pre-eclampsia, parity, caesarean section, height of the child’s father, history of preterm birth, total energy intake, dietary folate intake and folic acid supplementation were adjusted for in the multivariable logistic regression model. In some previous studies, the results showed that high dosage of folic acid may be associated with an increased risk of SGA at birth(23,24,27). Therefore, we conducted a sensitivity analysis restricted to participants with <24 weeks of gestational of age. We assessed goodness-of-fit of the models in the present study and all showed a good fit by the Homser–Lemeshow test (P > 0·05). All analyses were performed using the statistical software package SAS version 9.4.
Results
Table 1 shows the distributions of selected characteristics in participants with SGA and AGA births. Women who had SGA births were more likely to be younger than 25 years old, have less family income and lower education level, be exposed to smoke, be unemployed during pregnancy, have a lower pre-pregnancy BMI, gain less weight during pregnancy, be diagnosed with pre-eclampsia, be multipara, undergo caesarean delivery and have a history of preterm birth. Paternal height of SGA was more likely to be shorter than 175 cm compared with paternal height of AGA. Distributions of alcohol drinking during pregnancy, gestational diabetes and infant’s gender were similar between SGA and AGA.
Table 1.
Distributions of selected participant characteristics, according to small-for-gestational-age (SGA) and appropriate-for-gestational-age (AGA) births, among women (n 8758) and their children enrolled in the 2010–2012 Gansu Provincial Maternity and Child Care Hospital birth cohort, Lanzhou, China
| Characteristic | AGA (n 7974) | SGA (n 784) | P value | ||
|---|---|---|---|---|---|
| n | % | n | % | ||
| Maternal age (years) | |||||
| <25 | 1812 | 88·78 | 229 | 11·22 | <0·001 |
| 25–29 | 3330 | 92·35 | 276 | 7·65 | |
| ≥30 | 2832 | 91·03 | 279 | 8·97 | |
| Monthly income per capita (RMB) | |||||
| <3000 | 4193 | 89·48 | 493 | 10·52 | <0·001 |
| ≥3000 | 3781 | 92·85 | 291 | 7·15 | |
| Maternal education level | |||||
| <College | 4903 | 89·67 | 565 | 10·33 | <0·001 |
| ≥College | 3071 | 93·34 | 219 | 6·66 | |
| Smoking (passive and active) | |||||
| No | 6430 | 91·65 | 586 | 8·35 | <0·001 |
| Yes | 1544 | 88·63 | 198 | 11·37 | |
| Drink during pregnancy | |||||
| No | 7959 | 91·05 | 782 | 8·95 | 0·684* |
| Yes | 15 | 88·24 | 2 | 11·76 | |
| Maternal employment | |||||
| No | 3812 | 89·69 | 438 | 10·31 | <0·001 |
| Yes | 4162 | 92·32 | 346 | 7·68 | |
| Pre-pregnancy BMI (kg/m2) | |||||
| <18·5 | 1724 | 89·70 | 198 | 10·30 | 0·005 |
| 18·5–23·9 | 5292 | 91·76 | 475 | 8·24 | |
| ≥24·0 | 958 | 89·62 | 111 | 10·38 | |
| Weight gain during pregnancy (kg) | |||||
| <15 | 2524 | 88·34 | 333 | 11·66 | <0·001 |
| 15–18·5 | 2560 | 91·72 | 231 | 8·28 | |
| >18·5 | 2890 | 92·93 | 220 | 7·07 | |
| Pre-eclampsia | |||||
| No | 7768 | 91·87 | 687 | 8·13 | <0·001 |
| Yes | 206 | 67·99 | 97 | 32·01 | |
| Gestational diabetes | |||||
| No | 7913 | 91·02 | 781 | 8·98 | 0·230 |
| Yes | 61 | 95·31 | 3 | 4·69 | |
| Parity | |||||
| Primipara | 5826 | 91·59 | 535 | 8·41 | 0·027 |
| Multipara | 2148 | 89·61 | 249 | 10·39 | |
| Caesarean section | |||||
| No | 5164 | 92·30 | 431 | 7·70 | <0·001 |
| Yes | 2810 | 88·84 | 353 | 11·16 | |
| History of preterm birth | |||||
| No | 7920 | 91·12 | 772 | 8·88 | 0·008 |
| Yes | 54 | 81·82 | 12 | 18·18 | |
| Height of child’s father (cm) | |||||
| ≤175 | 5416 | 90·43 | 573 | 9·57 | 0·003 |
| >175 | 2558 | 92·38 | 211 | 7·62 | |
| Gender of live birth | |||||
| Male | 4186 | 91·52 | 388 | 8·48 | 0·108 |
| Female | 3788 | 90·54 | 396 | 9·46 | |
Fisher’s exact test.
Compared with non-users, folic acid supplement users had a reduced risk of SGA (OR = 0·72, 95 % CI 0·60, 0·86; Table 2). The significant reduced odds was seen mainly for those who had used folic acid supplements for more than 12 weeks (OR = 0·78, 95 % CI 0·70, 0·87, P trend < 0·001; OR = 0·962, 95 % CI 0·945, 0·979 per 2-week increase in folic acid supplement use). After stratifying by time period of use, significant associations were observed for those who took supplements before conception and during pregnancy (OR = 0·58, 95 % CI 0·46, 0·75, P trend < 0·001; OR = 0·955, 95 % CI 0·935, 0·976 per 2-week increase) or during pregnancy only (OR = 0·78, 95 % CI 0·65, 0·94, P trend = 0·002; OR = 0·966, 95 % CI 0·942, 0·991 per 2-week increase). No significant association was observed among women who took supplements before conception only. We did not observe significant associations between dietary folate intake and SGA. In addition, we assessed the joint effect of folic acid supplementation and dietary folate intake on SGA and found no significant interaction between them (P interaction = 0·062). Additionally, in the sensitivity analysis restricted to the participants with <24 weeks of gestational age (see online supplementary material, Supplemental Table S1), the results were consistent with the findings of Table 2; significant associations also were observed for those who took supplements before conception and during pregnancy (OR = 0·62, 95 % CI 0·48, 0·81, P trend < 0·001; OR = 0·953, 95 % CI 0·927, 0·980 per 2-week increase).
Table 2.
Associations of folic acid supplementation and dietary folate intake with the risk of small for gestational age (SGA) among women (n 8758) and their children enrolled in the 2010–2012 Gansu Provincial Maternity and Child Care Hospital birth cohort, Lanzhou, China
| Folic acid/folate intake duration | AGA (n 7974) | SGA (n 784) | OR | 95 % CI | OR* | 95 % CI |
|---|---|---|---|---|---|---|
| Folic acid supplement | ||||||
| Non-users | 1762 | 252 | 1·00 | Ref. | 1·00 | Ref. |
| Users | 6212 | 532 | 0·60 | 0·51, 0·70 | 0·72 | 0·60, 0·86 |
| ≤12 weeks | 3239 | 321 | 0·69 | 0·58, 0·83 | 0·79 | 0·66, 0·96 |
| >12 weeks | 2973 | 211 | 0·70 | 0·64, 0·78 | 0·78 | 0·70, 0·87 |
| P trend | <0·001 | <0·001 | ||||
| Per 2-week increase | 0·943 | 0·928, 0·959 | 0·962 | 0·945, 0·979 | ||
| Before conception and during pregnancy | 2344 | 155 | 0·46 | 0·37, 0·57 | 0·58 | 0·46, 0·75 |
| <24 weeks | 1101 | 78 | 0·50 | 0·38, 0·65 | 0·61 | 0·45, 0·81 |
| ≥24 weeks | 1243 | 77 | 0·66 | 0·58, 0·75 | 0·77 | 0·66, 0·90 |
| P trend | <0·001 | <0·001 | ||||
| Per 2-week increase | 0·937 | 0·919, 0·954 | 0·955 | 0·935, 0·976 | ||
| Before conception only | 294 | 28 | 0·67 | 0·44, 1·01 | 0·84 | 0·54, 1·29 |
| ≤8 weeks | 123 | 12 | 0·68 | 0·37, 1·25 | 0·76 | 0·41, 1·41 |
| >8 weeks | 171 | 16 | 0·81 | 0·62, 1·05 | 0·95 | 0·72, 1·26 |
| P trend | 0·061 | 0·471 | ||||
| Per 2-week increase | 0·936 | 0·870, 1·008 | 0·975 | 0·904, 1·052 | ||
| During pregnancy only | 3574 | 349 | 0·68 | 0·58, 0·81 | 0·78 | 0·65, 0·94 |
| ≤8 weeks | 1415 | 164 | 0·81 | 0·66, 1·00 | 0·85 | 0·68, 1·06 |
| >8 weeks | 2159 | 185 | 0·77 | 0·70, 0·86 | 0·85 | 0·76, 0·95 |
| P trend | <0·001 | 0·002 | ||||
| Per 2-week increase | 0·948 | 0·925, 0·970 | 0·966 | 0·942, 0·991 | ||
| Dietary folate intake (µg/d) | ||||||
| Before pregnancy | ||||||
| Q1 < 115·63 | 1963 | 232 | 1·00 | Ref. | 1·00 | Ref. |
| Q2 = 115·63–157·42 | 2002 | 157 | 0·66 | 0·54, 0·82 | 0·78 | 0·62, 1·00 |
| Q3 = 157·42–218·91 | 1959 | 206 | 0·94 | 0·85, 1·04 | 1·04 | 0·93, 1·16 |
| Q4 ≥ 218·91 | 2050 | 189 | 0·92 | 0·86, 1·00 | 1·02 | 0·93, 1·12 |
| P trend | 0·124 | 0·311 | ||||
| Per 10-μg increase | 0·996 | 0·982, 1·010 | 1·014 | 0·997, 1·031 | ||
| During pregnancy | ||||||
| Q1 < 149·47 | 1971 | 240 | 1·00 | Ref. | 1·00 | Ref. |
| Q2 = 149·47–197·56 | 1967 | 185 | 0·77 | 0·63, 0·95 | 0·92 | 0·73, 1·15 |
| Q3 = 197·56–263·21 | 1988 | 166 | 0·83 | 0·75, 0·92 | 0·97 | 0·86, 1·10 |
| Q4 ≥ 263·21 | 2048 | 193 | 0·92 | 0·86, 0·98 | 1·06 | 0·96, 1·18 |
| P trend | 0·005 | 0·418 | ||||
| Per 10-μg increase | 0·985 | 0·971, 0·998 | 1·012 | 0·995, 1·030 | ||
AGA, appropriate for gestational age; Q, quartile; ref., reference category.
Multiplicative interaction on SGA: OR = 0·85 (95 % CI 0·71, 1·01), P = 0·062.
OR adjusted for maternal age, monthly income per capita, maternal education level, smoking, maternal employment, pre-pregnancy BMI, weight gain during pregnancy, pre-eclampsia, parity, caesarean section, height of child’s father, history of preterm birth, total energy intake, dietary folate intake or folic acid supplement.
We then analysed the data separately for SGA at ≥37 weeks of gestational age and SGA at <37 weeks of gestational age (Table 3). Significant protective effects of folic acid supplement use were seen on SGA at ≥37 weeks of gestational age (OR = 0·70, 95 % CI 0·58, 0·85 for users; OR = 0·59, 95 % CI 0·45, 0·77 for use before conception and during pregnancy, with OR = 0·953, 95 % CI 0·931, 0·976 per 2-week increase in folic acid supplement use; and OR = 0·76, 95 % CI 0·62, 0·93 for use during pregnancy only, with OR = 0·968, 95 % CI 0·941, 0·994 per 2-week increase) and no significant association was observed among women who took supplements before conception only. We did not observe significant associations between folic acid supplement use and SGA at <37 weeks of gestational age. Nor did we observe any significant associations of SGA at ≥37 weeks of gestational age and SGA at <37 weeks of gestational age with dietary folate intake. In addition, we assessed the joint effect of folic acid supplementation and dietary folate intake on SGA at ≥37 weeks of gestational age and SGA at <37 weeks of gestational age; there was no significant interaction on SGA at ≥37 weeks of gestational age (P interaction = 0·698) or on SGA at <37 weeks of gestational age (P interaction = 0·085).
Table 3.
Associations of folic acid supplementation and dietary folate intake with the risk of small for gestational age (SGA) at ≥37 weeks of gestational age and SGA at <37 weeks of gestational age among women (n 8758) and their children enrolled in the 2010–2012 Gansu Provincial Maternity and Child Care Hospital birth cohort, Lanzhou, China
| Folic acid/folate intake duration | SGA at ≥37 weeks of gestational age | SGA at <37 weeks of gestational age | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Controls (n 7211) |
Cases (n 632) |
OR | 95 % CI | OR* | 95 % CI | Controls (n 763) |
Cases (n 152) |
OR | 95 % CI | OR* | 95 % CI | |
| Folic acid supplement | ||||||||||||
| Non-users | 1531 | 197 | 1·00 | Ref. | 1·00 | Ref. | 231 | 55 | 1·00 | Ref. | 1·00 | Ref. |
| Users | 5680 | 435 | 0·60 | 0·50, 0·71 | 0·70 | 0·58, 0·85 | 532 | 97 | 0·77 | 0·48, 1·25 | 0·76 | 0·53, 1·10 |
| ≤12 weeks | 2901 | 258 | 0·69 | 0·57, 0·84 | 0·79 | 0·64, 0·97 | 338 | 63 | 0·78 | 0·53, 1·17 | 0·77 | 0·49, 1·21 |
| >12 weeks | 2779 | 177 | 0·70 | 0·63, 0·78 | 0·77 | 0·68, 0·87 | 194 | 34 | 0·86 | 0·68, 1·08 | 0·79 | 0·59, 1·06 |
| P trend | <0·001 | <0·001 | 0·177 | 0·137 | ||||||||
| Per 2-week increase | 0·944 | 0·927, 0·961 | 0·961 | 0·943, 0·980 | 0·971 | 0·935, 1·010 | 0·966 | 0·925, 1·011 | ||||
| Before conception and during pregnancy | 2173 | 128 | 0·46 | 0·36, 0·58 | 0·59 | 0·45, 0·77 | 171 | 27 | 0·66 | 0·40, 1·10 | 0·58 | 0·32, 1·07 |
| <24 weeks | 1007 | 65 | 0·50 | 0·38, 0·67 | 0·64 | 0·46, 0·88 | 94 | 13 | 0·58 | 0·30, 1·11 | 0·47 | 0·23, 1·00 |
| ≥24 weeks | 1166 | 63 | 0·65 | 0·56, 0·75 | 0·76 | 0·64, 0·91 | 77 | 14 | 0·87 | 0·63, 1·20 | 0·84 | 0·58, 1·24 |
| P trend | <0·001 | <0·001 | 0·221 | 0·271 | ||||||||
| Per 2-week increase | 0·936 | 0·917, 0·955 | 0·953 | 0·931, 0·976 | 0·970 | 0·929, 1·015 | 0·971 | 0·921, 1·023 | ||||
| Before conception only | 266 | 25 | 0·73 | 0·47, 1·13 | 0·92 | 0·58, 1·45 | 28 | 3 | 0·45 | 0·13, 1·53 | 0·38 | 0·11, 1·47 |
| During pregnancy only | 3241 | 282 | 0·68 | 0·56, 0·82 | 0·76 | 0·62, 0·93 | 333 | 67 | 0·85 | 0·57, 1·25 | 0·80 | 0·50, 1·26 |
| ≤8 weeks | 1252 | 124 | 0·77 | 0·61, 0·98 | 0·81 | 0·64, 1·04 | 163 | 40 | 1·03 | 0·66, 1·62 | 0·95 | 0·56, 1·59 |
| >8 weeks | 1989 | 158 | 0·79 | 0·70, 0·88 | 0·86 | 0·76, 0·96 | 170 | 27 | 0·82 | 0·64, 1·05 | 0·77 | 0·57, 1·03 |
| P trend | <0·001 | 0·006 | 0·144 | 0·080 | ||||||||
| Per 2-week increase | 0·950 | 0·926, 0·975 | 0·968 | 0·941, 0·994 | 0·968 | 0·914, 1·026 | 0·947 | 0·887, 1·010 | ||||
| Dietary folate intake (µg/d) | ||||||||||||
| Before pregnancy | ||||||||||||
| Q1 < 115·63 | 1736 | 172 | 1·00 | Ref. | 1·00 | Ref. | 227 | 60 | 1·00 | Ref. | 1·00 | Ref. |
| Q2 = 115·63–157·42 | 1815 | 128 | 0·71 | 0·56, 0·91 | 0·83 | 0·65, 1·07 | 187 | 29 | 0·59 | 0·36, 0·96 | 0·59 | 0·32, 1·06 |
| Q3 = 157·42–218·91 | 1773 | 173 | 0·99 | 0·89, 1·11 | 1·09 | 0·97, 1·23 | 186 | 33 | 0·82 | 0·65, 1·04 | 0·93 | 0·70, 1·24 |
| Q4 ≥ 218·91 | 1887 | 159 | 0·95 | 0·88, 1·02 | 1·03 | 0·93, 1·13 | 163 | 30 | 0·89 | 0·76, 1·05 | 1·02 | 0·81, 1·29 |
| P trend | 0·609 | 0·194 | 0·119 | 0·718 | ||||||||
| Per 10-μg increase | 1·001 | 0·986, 1·016 | 1·020 | 0·993, 1·037 | 0·982 | 0·948, 1·017 | 0·992 | 0·953, 1·036 | ||||
| During pregnancy | ||||||||||||
| Q1 < 149·47 | 1705 | 183 | 1·00 | Ref. | 1·00 | Ref. | 266 | 57 | 1·00 | Ref. | 1·00 | Ref. |
| Q2 = 149·47–197·56 | 1788 | 146 | 0·76 | 0·61, 0·96 | 0·84 | 0·65, 1·09 | 179 | 39 | 1·02 | 0·65, 1·59 | 1·29 | 0·75, 2·24 |
| Q3 = 197·56–263·21 | 1822 | 135 | 0·83 | 0·74, 0·93 | 0·98 | 0·86, 1·12 | 166 | 31 | 0·93 | 0·74, 1·19 | 0·99 | 0·72, 1·37 |
| Q4 ≥ 263·21 | 1896 | 168 | 0·94 | 0·87, 1·00 | 1·04 | 0·93, 1·17 | 152 | 25 | 0·92 | 0·77, 1·09 | 1·35 | 0·99, 2·.01 |
| P trend | 0·063 | 0·369 | 0·276 | 0·951 | ||||||||
| Per 10-μg increase | 0·992 | 0·977, 1·007 | 1·018 | 0·998, 1·036 | 0·976 | 0·943, 1·010 | 0·988 | 0·944, 1·034 | ||||
Q, quartile; ref., reference category.
Multiplicative interaction on SGA at ≥37 weeks of gestational age: OR = 1·04 (95 % CI 0·84, 1·29), P = 0·698.
Multiplicative interaction on SGA at <37 weeks of gestational age: OR = 0·64 (95 % CI 0·38, 1·06), P = 0·085.
OR adjusted for maternal age, monthly income per capita, education level, smoking, employment, pre-pregnancy BMI, weight gain during pregnancy, pre-eclampsia, parity, caesarean section, height of child’s father, history of preterm birth, total energy intake, dietary folate intake or folic acid supplement.
We further stratified the analysis by parity (Table 4). A significant protective effect on nulliparous SGA was observed among folic acid supplementation users (OR = 0·67, 95 % CI 0·54, 0·84), but folic acid supplementation was not related to multiparous SGA (OR = 0·80, 95 % CI 0·61, 1·06). The significant protective effect on nulliparous SGA was observed for those who took supplements before conception and during pregnancy (OR = 0·62, 95 % CI 0·46, 0·83, P trend = 0·002; OR = 0·960, 95 % CI 0·937, 0·984 per 2-week increase in folic acid supplement use) and during pregnancy only (OR = 0·71, 95 % CI 0·56, 0·90, P trend = 0·002; OR = 0·969, 95 % CI 0·941, 0·998 per 2-week increase), but not before conception only (OR = 0·85, 95 % CI 0·51, 1·41). We did not observe significant associations of nulliparous SGA and multiparous SGA with dietary folate intake. In addition, we assessed the joint effect of folic acid supplementation and dietary folate intake on nulliparous SGA and multiparous SGA, and found no significant interaction (nulliparous SGA: P interaction = 0·228, multiparous SGA: P interaction = 0·174).
Table 4.
Associations of folic acid supplementation and dietary folate intake with the risk of nulliparous small for gestational age (SGA) and multiparous SGA among women (n 8758) and their children enrolled in the 2010–2012 Gansu Provincial Maternity and Child Care Hospital birth cohort, Lanzhou, China
| Folic acid/folate intake duration | Nulliparous SGA | Multiparous SGA | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Controls (n 5826) |
Cases (n 535) |
OR | 95 % CI | OR* | 95 % CI | Controls (n 2148) |
Cases (n 249) |
OR | 95 % CI | OR* | 95 % CI | |
| Folic acid supplement | ||||||||||||
| Non-users | 982 | 138 | 1·00 | Ref. | 1·00 | Ref. | 780 | 114 | 1·00 | Ref. | 1·00 | Ref. |
| Users | 4844 | 397 | 0·58 | 0·48, 0·72 | 0·67 | 0·54, 0·84 | 1368 | 135 | 0·68 | 0·52, 0·88 | 0·80 | 0·61, 1·06 |
| ≤12 weeks | 2379 | 224 | 0·67 | 0·54, 0·84 | 0·73 | 0·58, 0·93 | 860 | 97 | 0·77 | 0·58, 1·03 | 0·90 | 0·67, 1·22 |
| >12 weeks | 2465 | 173 | 0·71 | 0·63, 0·80 | 0·79 | 0·69, 0·91 | 508 | 38 | 0·72 | 0·59, 0·87 | 0·77 | 0·63, 0·95 |
| P trend | <0·001 | <0·001 | <0·001 | 0·022 | ||||||||
| Per 2-week increase | 0·949 | 0·932, 0·967 | 0·965 | 0·946, 0·985 | 0·933 | 0·902, 0·966 | 0·951 | 0·917, 0·985 | ||||
| Before conception and during pregnancy | 1971 | 132 | 0·48 | 0·37, 0·61 | 0·62 | 0·46, 0·83 | 373 | 23 | 0·42 | 0·27, 0·67 | 0·52 | 0·32, 0·85 |
| <24 weeks | 889 | 64 | 0·51 | 0·35, 0·70 | 0·64 | 0·45, 0·91 | 212 | 14 | 0·62 | 0·44, 0·88 | 0·70 | 0·48, 1·01 |
| ≥24 weeks | 1082 | 68 | 0·67 | 0·58, 0·78 | 0·79 | 0·66, 0·94 | 161 | 9 | 0·45 | 0·25, 0·80 | 0·53 | 0·29, 0·97 |
| P trend | <0·001 | 0·002 | <0·001 | 0·010 | ||||||||
| Per 2-week increase | 0·941 | 0·921, 0·962 | 0·960 | 0·937, 0·984 | 0·920 | 0·879, 0·964 | 0·938 | 0·893, 0·986 | ||||
| Before conception only | 224 | 21 | 0·67 | 0·41, 1·08 | 0·85 | 0·51, 1·41 | 70 | 7 | 0·68 | 0·31, 1·53 | 0·83 | 0·36, 1·88 |
| During pregnancy only | 2649 | 244 | 0·66 | 0·53, 0·82 | 0·71 | 0·56, 0·90 | 925 | 105 | 0·78 | 0·59, 1·03 | 0·90 | 0·67, 1·21 |
| ≤8 weeks | 983 | 110 | 0·80 | 0·61, 1·04 | 0·78 | 0·59, 1·03 | 432 | 54 | 0·86 | 0·61, 1·21 | 0·97 | 0·68, 1·39 |
| >8 weeks | 1666 | 134 | 0·76 | 0·67, 0·86 | 0·83 | 0·72, 0·95 | 493 | 51 | 0·71 | 0·50, 1·01 | 0·79 | 0·54, 1·14 |
| P trend | <0·001 | 0·002 | 0·051 | 0·265 | ||||||||
| Per 2-week increase | 0·954 | 0·925, 0·982 | 0·969 | 0·941, 0·998 | 0·943 | 0·902, 0·996 | 0·958 | 0·915, 1·004 | ||||
| Dietary folate intake (µg/d) | ||||||||||||
| Before pregnancy | ||||||||||||
| Q1 < 115·63 | 1399 | 147 | 1·00 | Ref. | 1·00 | Ref. | 564 | 85 | 1·00 | Ref. | 1·00 | Ref. |
| Q2 = 115·63–157·42 | 1523 | 122 | 0·76 | 0·59, 0·99 | 0·90 | 0·68, 1·18 | 479 | 35 | 0·49 | 0·32, 0·73 | 0·56 | 0·36, 0·90 |
| Q3 = 157·42–218·91 | 1450 | 139 | 0·96 | 0·85, 1·08 | 1·07 | 0·94, 1·23 | 509 | 67 | 0·94 | 0·79, 1·11 | 0·96 | 0·79, 1·17 |
| Q4 ≥ 218·91 | 1454 | 127 | 0·94 | 0·87, 1·02 | 1·02 | 0·91, 1·14 | 596 | 62 | 0·88 | 0·79, 0·99 | 1·02 | 0·87, 1·19 |
| P trend | 0·338 | 0·440 | 0·182 | 0·713 | ||||||||
| Per 10-μg increase | 1·004 | 0·988, 1·021 | 1·022 | 1·004, 1·041 | 0·979 | 0·956, 1·003 | 1·001 | 0·973, 1·031 | ||||
| During pregnancy | ||||||||||||
| Q1 < 149·47 | 1294 | 156 | 1·00 | Ref. | 1·00 | Ref. | 677 | 84 | 1·00 | Ref. | 1·00 | Ref. |
| Q2 = 149·47–197·56 | 1484 | 127 | 0·71 | 0·56, 0·91 | 0·85 | 0·65, 1·12 | 483 | 58 | 0·97 | 0·68, 1·38 | 1·08 | 0·73, 1·60 |
| Q3 = 197·56–263·21 | 1560 | 113 | 0·78 | 0·68, 0·88 | 0·90 | 0·78, 1·05 | 428 | 53 | 0·99 | 0·83, 1·20 | 1·18 | 0·95, 1·47 |
| Q4 ≥ 263·21 | 1488 | 139 | 0·92 | 0·85, 0·99 | 1·06 | 0·93, 1·20 | 560 | 54 | 0·92 | 0·82, 1·04 | 1·11 | 0·92, 1·34 |
| P trend | 0·019 | 0·794 | 0·223 | 0·373 | ||||||||
| Per 10-μg increase | 0·992 | 0·975, 1·009 | 1·019 | 0·998, 1·040 | 0·975 | 0·952, 0·999 | 0·999 | 0·969, 1·030 | ||||
Q, quartile; ref., reference category.
Multiplicative interaction on nulliparous SGA: OR = 0·88 (95 % CI 0·72, 1·08), P = 0·228.
Multiplicative interaction on multiparous-SGA: OR = 0·79 (95 % CI 0·56, 1·11), P = 0·174.
OR adjusted for maternal age, monthly income per capita, education level, smoking, employment, pre-pregnancy BMI, weight gain during pregnancy, pre-eclampsia, caesarean section, height of child’s father, history of preterm, total energy intake, dietary folate intake or folic acid supplement.
Discussion
To our knowledge, our study represents the first one to concurrently examine folic acid supplementation and dietary folate intake in relation to the risk of SGA. Our study found that folic acid supplementation was associated with a reduced risk of SGA and the protective association was seen mainly for term births and births from nulliparous mothers. No significant interaction between folic acid supplementation and dietary folate intake was found for SGA, SGA at <37 weeks of gestational age, SGA at ≥37 weeks of gestational age, nulliparous SGA and multiparous SGA.
Earlier studies investigating the associations of folic acid supplementation and dietary folate intake with SGA provided conflicting results(12–22,25–30). Variations in dosage of folic acid used, time periods and duration of folic acid supplementation, definition of SGA, and lack of consideration of effects of preterm birth and parity on SGA among different populations might partially contribute to the inconsistent results.
Our study found that folic acid supplementation was associated with a reduced risk of SGA overall. However, when data were stratified by time periods of folic acid supplementation, significant associations were observed for those who took supplements before conception and during pregnancy or during pregnancy only, but not for those taking supplements before conception only, which was consistent with some previous studies(13,17–22,26,28,30,33). The magnitude of the protective effect of folic acid supplementation seemed greater for taking supplements before conception and during pregnancy compared with taking supplements during pregnancy only. While the strengthened association could be due to longer duration of intake, there are possible biological mechanisms that could explain the observed association. Folate is an essential ingredient in the synthesis of methionine and subsequent S-adenosylmethionine, which plays a key role in DNA methylation(52). The epigenome is particularly susceptible during the early stages of embryogenesis(53). Abnormal folate concentration may cause epigenetic modifications and subsequently result in altered placental and fetal growth patterns(54–58). Furthermore, folate plays a critical role in protein and DNA synthesis(36,54). Sufficient folate intake promotes a cell-rich placenta(59), which is beneficial later in the quantitatively important growth phase of the fetus, and insufficient folate intake during pregnancy will lead concentrations of folate in maternal plasma and erythrocytes to decrease from the fifth month of pregnancy onwards(60). Significant fetal growth takes place during the later second trimester and the third trimester, therefore a sufficient supply of folate during pregnancy is critical to maintain normal fetal growth(61–63).
Our study found that folic acid supplementation was associated with SGA at ≥37 weeks of gestational age but not with SGA at <37 weeks of gestational age, suggesting that SGA at ≥37 weeks of gestational age and SGA at <37 weeks of gestational age are two different types of SGA and might have different aetiological profiles. To our knowledge, the current study is the first investigating the relationship between SGA at ≥37 weeks of gestational age and folic acid supplementation, and the second study investigating the relationship between SGA at <37 weeks of gestational age and folic acid supplementation, but Chen et al. found that taking folic acid supplementation more than 3 months before pregnancy was associated with a significant reduction in incidence of preterm SGA(16). So, the biological processes should be studied further.
Our study also found a significant protective effect of folic acid supplementation on nulliparous SGA but not multiparous SGA. It has been suggested that multiparous women offer a more favourable environment for placental and fetal growth by remodelling of the maternal vascular structure in former pregnancies(36,55), which may attenuate the effect of folic acid supplementation on fetal growth. Actually, the biological processes should also be further studied.
Limitations should be considered when interpreting the study results. Information on folic acid supplementation and dietary folate intake was based on self-report, thus potential recall bias was unavoidable. Since the benefit of folic acid on SGA was not well established, there was unlikely differential recall bias associated with SGA. A strong correlation between self-reported folate intake and serum folate concentrations during pregnancy has been suggested(63). While we collected detailed information on potential confounding variables, residual confounding resulting from unknown sources cannot be ruled out. Although our study had relatively large sample size, limited statistical power was presented for stratified analysis, particularly for SGA at <37 weeks of gestational age. In addition, the representation of the sample in the study was limited because the participation rate was 74 %.
Conclusion
In conclusion, our study suggested a protective effect of folic acid supplementation on risk of SGA and the protective effect varied by preterm status and parity. Future studies are warranted to replicate the findings.
Acknowledgements
Acknowledgements: The authors thank all the study personnel from the GPMCCH for their exceptional efforts on study subject recruitment. Financial support: This study was financially supported by internal funding from the GPMCCH and the National Institutes of Health (grant numbers K02HD70324 and R01ES019587). The funders had no role in the design, analysis or writing of this article. Conflict of interest: None. Authorship: Y.Z. and J.Q. designed the birth cohort study; H.G., B.M. and M.W. analysed data and wrote the paper; H.G., B.M., M.W., Q.L., L.Y., Y.X., Y.W., X.H., H.C., X.L., L.L., M.Z. and X.X. conducted this research; Y.Z. and H.G. had primary responsibility for the final content. All authors read and approved the final manuscript. Ethics of human subject participation: This study was conducted according to the guidelines laid down in the Declaration of Helsinki and all procedures involving research study participants were approved by the human investigation committees at the GPMCCH and Yale University. Written informed consent was obtained from all subjects.
Supplementary material
For supplementary material accompanying this paper visit https://doi.org/10.1017/S1368980019003331.
click here to view supplementary material
References
- 1. McIntire DD, Bloom SL, Casey BM et al. (1999) Birth weight in relation to morbidity and mortality among newborn infants. N Engl J Med 340, 1234–1238. [DOI] [PubMed] [Google Scholar]
- 2. Pallotto EK & Kilbride HW (2006) Perinatal outcome and later implications of intrauterine growth restriction. Clin Obstet Gynecol 49, 257–269. [DOI] [PubMed] [Google Scholar]
- 3. Hack M, Taylor HG, Drotar D et al. (2005) Chronic conditions, functional limitations, and special health care needs of school-aged children born with extremely low-birth-weight in the 1990s. JAMA 294, 318–325. [DOI] [PubMed] [Google Scholar]
- 4. Hille ET, den Ouden AL, Saigal S et al. (2001) Behavioural problems in children who weigh 1000 g or less at birth in four countries. Lancet 357, 1641–1643. [DOI] [PubMed] [Google Scholar]
- 5. Nafee TM, Farrell WE, Carroll WD et al. (2008) Epigenetic control of fetal gene expression. BJOG 115, 158–168. [DOI] [PubMed] [Google Scholar]
- 6. Schlotz W & Phillips DI (2009) Fetal origins of mental health: evidence and mechanisms. Brain Behav Immun 23, 905–916. [DOI] [PubMed] [Google Scholar]
- 7. Tosh DN, Fu Q, Callaway CW et al. (2010) Epigenetics of programmed obesity: alteration in IUGR rat hepatic IGF1 mRNA expression and histone structure in rapid vs. delayed postnatal catch-up growth. Am J Physiol Gastrointest Liver Physiol 299, G1023–G1029. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8. Waterland RA (2009) Is epigenetics an important link between early life events and adult disease? Horm Res 71, 13–16. [DOI] [PubMed] [Google Scholar]
- 9. Weinstock M (2005) The potential influence of maternal stress hormones on development and mental health of the offspring. Brain Behav Immun 19, 296–308. [DOI] [PubMed] [Google Scholar]
- 10. Lockwood CJ (2002) Predicting premature delivery – no easy task. N Engl J Med 346, 282–284. [DOI] [PubMed] [Google Scholar]
- 11. Krishnaswamy K & Madhavan Nair K (2001) Importance of folate in human nutrition. Br J Nutr 85, Suppl. 2, S115–S124. [DOI] [PubMed] [Google Scholar]
- 12. Baker BC, Mackie FL, Lean SC et al. (2017) Placental dysfunction is associated with altered microRNA expression in pregnant women with low folate status. Mol Nutr Food Res 61, 1600646. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13. Bergen NE, Jaddoe VW, Timmermans S et al. (2012) Homocysteine and folate concentrations in early pregnancy and the risk of adverse pregnancy outcomes: the Generation R Study. BJOG 119, 739–751. [DOI] [PubMed] [Google Scholar]
- 14. Bukowski R, Malone FD, Porter FT et al. (2009) Preconceptional folate supplementation and the risk of spontaneous preterm birth: a cohort study. PLoS Med 6, e1000061. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15. Catov JM, Bodnar LM, Olsen J et al. (2011) Periconceptional multivitamin use and risk of preterm or small-for-gestational-age births in the Danish National Birth Cohort. Am J Clin Nutr 94, 906–912. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16. Chen S, Zhu R, Zhu H et al. (2017) The prevalence and risk factors of preterm small-for-gestational-age infants: a population-based retrospective cohort study in rural Chinese population. BMC Pregnancy Childbirth 17, 237. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17. Dwarkanath P, Barzilay JR, Thomas T et al. (2013) High folate and low vitamin B-12 intakes during pregnancy are associated with small-for-gestational age infants in South Indian women: a prospective observational cohort study. Am J Clin Nutr 98, 1450–1458. [DOI] [PubMed] [Google Scholar]
- 18. Furness DL, Yasin N, Dekker GA et al. (2012) Maternal red blood cell folate concentration at 10–12 weeks gestation and pregnancy outcome. J Matern Fetal Neonatal Med 25, 1423–1427. [DOI] [PubMed] [Google Scholar]
- 19. Goldenberg RL, Tamura T, Cliver SP et al. (1992) Serum folate and fetal growth retardation: a matter of compliance? Obstet Gynecol 79, 719–722. [PubMed] [Google Scholar]
- 20. Hodgetts VA, Morris RK, Francis A et al. (2015) Effectiveness of folic acid supplementation in pregnancy on reducing the risk of small-for-gestational age neonates: a population study, systematic review and meta-analysis. BJOG 122, 478–490. [DOI] [PubMed] [Google Scholar]
- 21. Kim MW, Ahn KH, Ryu KJ et al. (2014) Preventive effects of folic acid supplementation on adverse maternal and fetal outcomes. PLoS One 9, e97273. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22. Li N, Li Z, Ye R et al. (2017) Impact of periconceptional folic acid supplementation on low birth weight and small-for-gestational-age infants in China: a large prospective cohort study. J Pediatr 187, 105–110. [DOI] [PubMed] [Google Scholar]
- 23. Navarrete-Munoz EM, Gimenez Monzo D, Garcia de La Hera M et al. (2010) Folic acid intake from diet and supplements in a population of pregnant women in Valencia, Spain. Med Clin (Barc) 135, 637–643. [DOI] [PubMed] [Google Scholar]
- 24. Navarrete-Munoz EM, Valera-Gran D, Garcia-de-la-Hera M et al. (2019) High doses of folic acid in the periconceptional period and risk of low weight for gestational age at birth in a population based cohort study. Eur J Nutr 58, 241–251. [DOI] [PubMed] [Google Scholar]
- 25. Nilsen RM, Vollset SE, Monsen AL et al. (2010) Infant birth size is not associated with maternal intake and status of folate during the second trimester in Norwegian pregnant women. J Nutr 140, 572–579. [DOI] [PubMed] [Google Scholar]
- 26. Papadopoulou E, Stratakis N, Roumeliotaki T et al. (2013) The effect of high doses of folic acid and iron supplementation in early-to-mid pregnancy on prematurity and fetal growth retardation: the mother–child cohort study in Crete, Greece (Rhea study). Eur J Nutr 52, 327–336. [DOI] [PubMed] [Google Scholar]
- 27. Pastor-Valero M, Navarrete-Munoz EM, Rebagliato M et al. (2011) Periconceptional folic acid supplementation and anthropometric measures at birth in a cohort of pregnant women in Valencia, Spain. Br J Nutr 105, 1352–1360. [DOI] [PubMed] [Google Scholar]
- 28. Rolschau J, Kristoffersen K, Ulrich M et al. (1999) The influence of folic acid supplement on the outcome of pregnancies in the county of Funen in Denmark. Part I. Eur J Obstet Gynecol Reprod Biol 87, 105–110. [PubMed] [Google Scholar]
- 29. Ronnenberg AG, Goldman MB, Chen D et al. (2002) Preconception homocysteine and B vitamin status and birth outcomes in Chinese women. Am J Clin Nutr 76, 1385–1391. [DOI] [PubMed] [Google Scholar]
- 30. Timmermans S, Jaddoe VW, Hofman A et al. (2009) Periconception folic acid supplementation, fetal growth and the risks of low birth weight and preterm birth: the Generation R Study. Br J Nutr 102, 777–785. [DOI] [PubMed] [Google Scholar]
- 31. Wang S, Ge X, Zhu B et al. (2016) Maternal continuing folic acid supplementation after the first trimester of pregnancy increased the risk of large-for-gestational-age birth: a population-based birth cohort study. Nutrients 8, 493. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 32. Yan SQ, Xu YQ, Su PY et al. (2013) Relationship between folic acid supplements during peri-conceptional period and the adverse pregnancy outcomes: a cohort study. Zhonghua Liu Xing Bing Xue Za Zhi 34, 1–4. [PubMed] [Google Scholar]
- 33. Yang T, Gu Y, Wei X et al. (2017) Periconceptional folic acid supplementation and vitamin B12 status in a cohort of Chinese early pregnancy women with the risk of adverse pregnancy outcomes. J Clin Biochem Nutr 60, 136–142. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 34. Zheng JS, Guan Y, Zhao Y et al. (2016) Pre-conceptional intake of folic acid supplements is inversely associated with risk of preterm birth and small-for-gestational-age birth: a prospective cohort study. Br J Nutr 115, 509–516. [DOI] [PubMed] [Google Scholar]
- 35. Policy and Research Team, Save the Children China Programme (n.d.) Laws and Policies for Maternal and Young Child Health Care in China. http://resourcecentre.savethechildren.se/sites/default/files/documents/3378.pdf (accessed November 2019).
- 36. Kloosterman GJ (1970) On intrauterine growth. Int J Gynecol Obstet 8, 895–912. [Google Scholar]
- 37. Shaw GM, Carmichael SL, Nelson V et al. (2004) Occurrence of low birthweight and preterm delivery among California infants before and after compulsory food fortification with folic acid. Public Health Rep 119, 170–173. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 38. de Bree A, van Dusseldorp M, Brouwer IA et al. (1997) Folate intake in Europe: recommended, actual and desired intake. Eur J Clin Nutr 51, 643–660. [DOI] [PubMed] [Google Scholar]
- 39. Scholl TO & Johnson WG (2000) Folic acid: influence on the outcome of pregnancy. Am J Clin Nutr 71, 5 Suppl., 1295S–1303S. [DOI] [PubMed] [Google Scholar]
- 40. World Health Organization & Food and Agriculture Organization of the United Nations (2004) Folate and folic acid. In Vitamin and Mineral Requirements in Human Nutrition, 2nd ed., pp. 289–302. Rome: FAO. [Google Scholar]
- 41. Ren AG (2015) Prevention of neural tube defects with folic acid: the Chinese experience. World J Clin Pediatr 4, 41–44. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 42. Liu J, Jin L, Meng Q et al. (2015) Changes in folic acid supplementation behaviour among women of reproductive age after the implementation of a massive supplementation programme in China. Public Health Nutr 18, 582–588. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 43. Zeng Z & Zhu J (2010) Low folic acid supplement intake rate among women in northern China with a high-prevalence of neural tube defects, 2008. Prev Med 51, 338–339. [DOI] [PubMed] [Google Scholar]
- 44. Zhang L, Ren A, Li Z et al. (2006) Folate concentrations and folic acid supplementation among women in their first trimester of pregnancy in a rural area with a high prevalence of neural tube defects in Shanxi, China. Birth Defects Res A Clin Mol Teratol 76, 461–466. [DOI] [PubMed] [Google Scholar]
- 45. Liu X, Lv L, Zhang H et al. (2016) Folic acid supplementation, dietary folate intake and risk of preterm birth in China. Eur J Nutr 55, 1411–1422. [DOI] [PubMed] [Google Scholar]
- 46. Qiu J, He X, Cui H et al. (2014) Passive smoking and preterm birth in urban China. Am J Epidemiol 180, 94–102. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 47. Wang Y, Zhao N, Qiu J et al. (2015) Folic acid supplementation and dietary folate intake, and risk of preeclampsia. Eur J Clin Nutr 69, 1145–1150. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 48. Zhao N, Qiu J, Zhang Y et al. (2015) Ambient air pollutant PM10 and risk of preterm birth in Lanzhou, China. Environ Int 76, 71–77. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 49. Dai L, Deng C, Li Y et al. (2014) Birth weight reference percentiles for Chinese. PLoS One 9, e104779. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 50. Duryea EL, Hawkins JS, McIntire DD et al. (2014) A revised birth weight reference for the United States. Obstet Gynecol 124, 16–22. [DOI] [PubMed] [Google Scholar]
- 51. Institute of Nutrition and Food Hygiene, Chinese Academy of Preventive Medicine (1999) Table of Food Components (National Representative Values). Beijing: People’s Hygiene Press. [Google Scholar]
- 52. Tamura T & Picciano MF (2006) Folate and human reproduction. Am J Clin Nutr 83, 993–1016. [DOI] [PubMed] [Google Scholar]
- 53. Timmermans S, Jaddoe VW, Silva LM et al. (2011) Folic acid is positively associated with uteroplacental vascular resistance: the Generation R study. Nutr Metab Cardiovasc Dis 21, 54–61. [DOI] [PubMed] [Google Scholar]
- 54. Bailey LB & Gregory JF 3rd (1999) Folate metabolism and requirements. J Nutr 129, 779–782. [DOI] [PubMed] [Google Scholar]
- 55. Bleker OP, Buimer M, van der Post JA et al. (2006) Ted (G.J.) Kloosterman: on intrauterine growth. The significance of prenatal care. Studies on birth weight, placental weight and placental index. Placenta 27, 1052–1054. [DOI] [PubMed] [Google Scholar]
- 56. Pennisi E (2005) Environmental epigenomics meeting. Supplements restore gene function via methylation. Science 310, 1761. [DOI] [PubMed] [Google Scholar]
- 57. Waterland RA & Jirtle RL (2004) Early nutrition, epigenetic changes at transposons and imprinted genes, and enhanced susceptibility to adult chronic diseases. Nutrition 20, 63–68. [DOI] [PubMed] [Google Scholar]
- 58. Steegers-Theunissen RP & Steegers EA (2003) Nutrient–gene interactions in early pregnancy: a vascular hypothesis. Eur J Obstet Gynecol Reprod Biol 106, 115–117. [DOI] [PubMed] [Google Scholar]
- 59. Rolschau J (1978) A prospective study of the placental weight and content of protein, RNA and DNA. Acta Obstet Gynecol Scand 57, 28–43. [DOI] [PubMed] [Google Scholar]
- 60. Cikot RJ, Steegers-Theunissen RP, Thomas CM et al. (2001) Longitudinal vitamin and homocysteine levels in normal pregnancy. Br J Nutr 85, 49–58. [DOI] [PubMed] [Google Scholar]
- 61. Thaler CJ (2014) Folate metabolism and human reproduction. Geburtshilfe Frauenheilkd 74, 845–851. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 62. McPartlin J, Halligan A, Scott JM et al. (1993) Accelerated folate breakdown in pregnancy. Lancet 341, 148–149. [DOI] [PubMed] [Google Scholar]
- 63. Scholl TO, Hediger ML, Schall JI et al. (1996) Dietary and serum folate: their influence on the outcome of pregnancy. Am J Clin Nutr 63, 520–525. [DOI] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
For supplementary material accompanying this paper visit https://doi.org/10.1017/S1368980019003331.
click here to view supplementary material