Summary
Infant mortality is among the most important indicators of health and development in global communities. One of the causes of neonatal mortality is low birth weight. This study aims at evaluating the risk factors for LBW in infants. This study was carried out using a nested case-control study in rural areas of Kurdistan province in Western Iran in 2015. The selection of case and control groups was based on the nesting using the risk set sampling approach. In total, 182 and 364 subjects were selected for the case group and the control group respectively. Data analysis was performed using the Stata-12 software with the point and spatial estimation of OR using the conditional logistic regression method. The multivariate logistic regression analysis performed shows that the maternal gestational age, the mother’s health history during pregnancy, any medication abuses by the mother, any mental stress during pregnancy, are LBW risk factors (P < 0.05). Prevention of LBW is possible by identifying effective factors and performing appropriate interventions in infants with low birth weight.
Keywords: Low birth weight, Nested case control, Risk factors, Iran
Introduction
Birth weight is one of the most important factors in the development, survival, and future of the baby; it is one of the main determinants of future physical and brain development of the child and also a valid sign of intrauterine growth [1, 2]. Low birth weight (LBW) is defined by the World Health Organization (WHO) as any weight less than 2,500 g regardless of the age of the baby [3]. Every year around 20 million newborns (17% of live births) weigh less than 2,500 g and more than 90% of them are born in developing countries [2, 4, 5].
According to WHO in 2015, the prevalence of LBW around the globe was 15%. It was 13% in developing countries, 9% in the US, 6% in East Asia and the Pacific, 13% in Sub-Saharan Africa, and 28% in South Asia [6].
LBW is closely related to infant mortality in the first days of life and even after infancy. It has been seen that the survival rate and survival chance of children who weigh less than 2,500 g after birth are much lower than other children [4, 7]. Generally, in these newborns the risk of neonatal mortality is 25-30 times more likely than those weighing more than 2,500 g, the lower the birth weight at birth, the greater the risk of neonatal mortality [5, 8]. It has been shown that LBW children who are alive with therapeutic interventions are two to three times more likely to suffer from short-term and long-term disabilities than other children [5, 9].
Many maternal and fetal factors are significantly associated with LBW [10-12]. Based on the results obtained from various studies, these factors include the mother’s age, occupation, weight, number of pregnancies, history of smoking, length of pregnancy, previous births, reproductive multiplication, inappropriate nutritional status, socioeconomic inequalities, lack of attention to proper diet and consumption of supplements during pregnancy, birth season, number of pregnancy cares and anemia, and birth defects, along with pre-pregnancy conditions and the socioeconomic status of the family related to LBW [5, 11]. LBW birth outcomes are high, especially in developing countries and the third world. Those who survive with LBW have cognitive and neurologic disorders as well as increased risk of hypertension, pulmonary disease, blood cholesterol, kidney damage, acute watery diarrhea, and immune system disorders [4]. Moreover, LBW is one of the determinants of neurological disorders and evolution, including backwardness and mental disability in learning, and may cause disorders relating to chronic diseases in adulthood [13].
Since LBW causes the risk of mortality, disability, and many diseases in childhood and even in adulthood while causing immense economic costs to the healthcare system and communities, it is very important to identify the factors affecting underweight during birth and hospital release [14, 15]. Even though Iran has been successful in reducing infant mortality over the past two decades, LBW is still recognized as one of the main causes of death and disability in this infant group. The purpose of this Study, which was conducted with a nested case-control study as a cost-effective [16] alternative to a cohort study, is to study the risk factors of LBW at present in a rural area of Iran.
Method
The nested case-control study was carried out in rural areas of Kurdistan province, Western Iran, for six months – from the beginning of December 2014 to the end of June 2015. In this study, the case and control groups were selected based on the design of the nest – i.e. with the risk set sampling approach.
ELIGIBILITY CRITERIA
The infants who were born in the study area with the birth weight 2,500 g or more were in the control group and infants with the birth weight less than 2,500 g were in the study group. Therefore, the criterion was birth weight and there were no other restrictions for being in the case and control groups.
In this study, in order to reach appropriate sample size in the study time frame, two infants with the birth weight 2,500 and more were included in the study as controls for each case of infants with the birth weight less than 2,500.
Data collection tools comprised a researcher-made checklist, which included independent variables and risk factors including maternal age, the mother’s education, maternal BMI, the number of pregnancies and previous births, newborn’s sex, birth season, the history of smoking for the mother, whether the mother is a second-hand smoker, the use of pregnancy supplement pills, the history of specific diseases during pregnancy, the history of drug use during pregnancy, parental separation history, the mother’s mental stress during pregnancy, baby birth rank, the number of pregnancy care, the kinship ratio of parents, the mother’s blood group, and possibility of having anemia, whether it is a natural pregnancy or IVF, and Rh maternal and neonatal conditions .
The data analysis for this study was performed using the Stata-12 software with a point estimate and OR (and CI) spacing that deals with raw and adapted conditional logistic regression and an error rate of less than 5%.
Logistic regression analysis on nesting case data means that the design is not identical in data analysis. If the exposure is constant over time, the odds ratio estimates the consistency ratio.
The conditional logistic regression method makes it possible to compare the cases and controls in each pair in which the case and control groups are defined as the outcome [17, 18].
FINDINGS
Mothers in the case and control groups had elementary education (49.8%), and about 90% of them were housewives. Most of the mothers did not have a kinship with their spouses, and more than 95% of them had no history of morbidity in their previous labors. The findings show that age, education, occupation, parents’ separation history, and maternal birth history has no significant relationship with their neonates in both case and control groups (P > 0.05) (Tab. I).
Tab. I.
Distribution of demographic variables and maternal history in case and control groups.
| Variable | Case N = 182 (%) |
Control N = 364 (%) |
Chi2 | P-value |
|---|---|---|---|---|
|
Mother’s age 20-35 years < 19 > 35 |
134 (74.1) 15 (7.48) 33 (18.03) |
273 (74.16) 29 (8.11) 62 (17.3) |
0.12 | 0.93 |
|
Maternal education
College education Diploma Guidance Elementary Illiterate |
3 (1.64) 34 (18.58) 30 (15.92) 88 (49.87) 27 (14.75) |
12 (3.31) 53 (14.6) 70 (19.16) 180 (49.77) 49 (13.50) |
3.18 | 0.52 |
|
Mother’s occupation Housewife Employee Laborer Other |
164 (90.01) 1 (0.55) 16 (8.74) 1 (0.55) |
322 (88.39) 10 (2.75) 28 (7.71) 4 (1.10) |
3.52 | 0.31 |
|
Parental relationship ratio No Yes |
164 (89.3) 18 (10.7) |
334 (92.19) 30 (7.81) |
2.77 | 0.56 |
|
History of parents’ separation (separation of living place) No Yes |
170 (93.44) 12 (6.56) |
346 (95.04) 18 (4.96) |
2.74 | 0.43 |
|
History of stillbirth Yes No |
174 (95.63) 8 (4.37) |
353 (96.97) 11 (3.03) |
3.1 | 0.54 |
The results of single-variable analysis indicate that the newborn’s sex and the history of abortion with the placement of the infant in the case and control groups did not have the required conditions for being in the multi-variable model (P > 0.2), while the gestational age, multiple pregnancy, maternal disease, history of hemorrhage, obsession, smoking, secondhand smoking, drug abuse, drug use under the supervision of the doctor, mental stress, and placement of the baby are required to be included in a multivariate model (P < 0.2) (Tab. II).
Tab. II.
Results of single-variable conditional logistic regression of newborns born in Kurdistan province (west of Iran).
| Variable | Cases N = 182 (%) |
Control N = 364 (%) |
Unadjusted OR (95% CI) | P-value |
|---|---|---|---|---|
|
Mother’s age years 20-35 < 19 > 35 |
134 (74.1) 15 (7.48) 33 (18.03) |
273 (74.16) 29 (8.11) 62 (17.3) |
1 1.05 (0.54-2.05) 1.07 (0.67-1.72) |
0.88 0.75 |
|
Maternal education
College education Diploma Guidance Elementary Elementary education Illiterate |
3 (1.64) 34 (18.58) 30 (15.92) 88 (49.87) 27 (14.75) |
12 (3.31) 53 (14.6) 70 (19.16) 180 (49.77) 49 (13.50) |
1 2.56 (0.67-9.70) 1.68 (0.44-6.33) 1.93 (0.53-6.93) 2.17 (0.57-8.22) |
0.16 0.43 0.31 0.25 |
|
Father’s education College education Diploma Guidance Elementary Elementary education Illiterate |
5 (2.73) 34 (19.26) 46 (25.14) 86 (46.99) 11 (6.01) |
17 (4.68) 85 (23.34) 93 (25.63) 147 (40.50) 22 (6.06) |
1 1.45 (0.48-4.3) 1.76 (0.59-5.24) 2.12 (0.72-6.20) 1.74 (0.49-6.10) |
0.50 0.30 0.16 0.38 |
|
Mother’s occupation Housewife Employee Laborer Other |
164 (90.01) 1 (0.55) 16 (8.74) 1 (0.55) |
322 (88.39) 10 (2.75) 28 (7.71) 4 (1.10) |
1 0.2 (0.02-1.56) 1.06 (0.57-1.98) 0.5 (0.05-4.50) |
0.12 0.83 0.53 |
|
Father’s occupation Employee Worker Self-employment Unemployed Other |
8 (4.37) 51 (27.87) 94 (51.01) 6 (3.28) 23 (13.37) |
19 (5.23) 111 (30.58) 190 (52.02) 9 (2.48) 35 (9.86) |
1 1.13 (0.43-3.93) 1.19 (0.48-3.95) 1.63 (0.48-6.39) 1.64 (0.58-4.63) |
0.79 0.70 0.49 0.43 |
|
Sex Girl Boy |
94 (51.6) 88 (48.35) |
192 (52.75) 172 (74.25) |
1 1.04 (0.73-1.48) |
0.79 |
|
Abortion history No Yes |
146 (79.78) 36 (20.22) |
298 (82.09) 66 (17.91) |
1 1.12 (0.7-1.8) |
0.61 |
|
Gestational weeks ≤ 37 > 37 |
76 (41.76) 106 (58.24) |
330 (90.91) 33 (9.09) |
1 18.2 (9.39-34.59) |
0.000 |
|
Distance between pregnancy 3 ≤ years 3 >years |
148 (81.32) 34 (18.68) |
317 (78.09) 47 (12.91) |
1 1.64 (0.97-2.7) |
0.06 |
|
Pregnancy Pregnancy1 Pregnancy2 2< Pregnancy |
72 (39.34) 54 (28.96) 56 (31.96) |
143 (39.39) 115 (31.96) 106 (28.65) |
1 0.94 (0.61-1.45) 1.05 (0.67-1.62) |
0.79 0.82 |
|
Childbirth 1 delivery 2 delivery 2 < delivery |
90 (49.18) 56 (30.05) 36 (20.77) |
167 (46.01) 114 (31.68) 83 (22.31) |
1 0.91 (0.61-1.63) 0.79 (0.49-1.27) |
0.66 0.34 |
|
Pregnancy with IVF No Yes |
178 (97.80) 4 (2.20) |
362 (99.45) 2 (0.55) |
1 4 (0.73-21.83) |
0.10 |
|
Multiple birth Singleton ≤ Twain |
136 (74.73) 46 (25.27) |
359 (98.63) 5 (1.37) |
1 4.45 (2.47-6.43) |
0.000 |
|
Abortion history No Yes |
146 (79.78) 36 (20.22) |
298 (82.09) 66 (17.91) |
1 1.12 (0.7-1.8) |
0.61 |
|
Mother’s disease history No Yes |
48 (26.52) 133 (73.84) |
250 (68.68) 114 (31.32) |
1 6.11 (3.95-9.45) |
0.000 |
|
History of stillbirth No Yes |
174 (95.63) 8 (4.37) |
353 (96.97) 11 (3.03) |
1 1.45 (0.58-3.61) |
0.42 |
|
History of bleeding No Yes |
154 (84.62) 28 (15.38) |
355 (97.53) 9 (2.47) |
1 7.59 (3.30-17.44) |
0.000 |
|
Birthday rating First birthday Second birthday ≤ third birthday |
77 (42.31) 62 (34.07) 43 (23.63) |
147 (40.38) 126 (34.62) 91(25) |
1 0.94 (0.36-1.41) 0.9 (0.57-1.41) |
0.78 0.65 |
|
Longing No Yes |
114 (62.64) 68 (37.36) |
255 (70.05) 109 (29.95) |
1 1.43 (0.96-2.13) |
0.07 |
|
blood group A B AB O |
58 (31.87) 52 (28.57) 16 (8.79) 56 (30.77) |
123 (33.79) 80 (21.98) 30 (8.24) 131 (35.99) |
1 1.36 (0.86-2.15) 1.12 (0.58-2.19) 0.9 (0.57-1.40) |
0.18 0.71 0.64 |
|
Smoking No Yes |
173 (95.05) 9 (4.95) |
361 (99.18) 3 (0.82) |
1 8.29 (1.77-38.69) |
0.007 |
|
The number of cares by the physician 3 ≤ care 2 care 1 care |
43 (23.62) 28 (15.38) 111 (60.99) |
75 (20.6) 77 (21.15) 212 (58.24) |
1 0.93 (0.56-1.51) 0.63 (0.35-1.13) |
0.77 0.12 |
|
The number of care by midwives 10 ≤ 9-5 care 5 > care |
6 (3.30) 57 (31.32) 119 (65.38) |
9 (2.20) 129 (35.54) 226 (62.29) |
1 0.55 (0.17-1.74) 0.72 (0.23-2.24) |
0.31 0.57 |
|
The number of care by health care ≤ 10 9-5 care < 5 care |
5 (2.75) 81 (44.51) 96 (52.75) |
6 (1.38) 256 (70.52) 102 (28.10) |
1 0.37 (0.1-1.31) 1.19 (0.32-4.33) |
0.79 0.12 |
|
Exposed to second smoke No Yes |
108 (59.34) 74 (40.66) |
293 (80.49) 71 (19.51) |
1 2.95 (1.94-4.48) |
0.000 |
|
Drug abuse No Yes |
142 (78.02) 40 (21.98) |
361 (99.18) 3 (0.82) |
39.25 (9.48-162.51) | 0.000 |
|
Drug use under medical supervision No Yes |
112 (61.54) 70 (38.46) |
325 (89.29) 39 (10.71) |
1 4.99 (3.12-7.99) |
0.000 |
|
Parental kinship relations No Yes |
164 (89.3) 18 (10.7) |
334 (92.19) 30 (7.81) |
1 1.19 (0.65-2.19) |
0.56 |
|
Separation of parent’s place of residence No Yes |
170 (93.44) 12 (6.55) |
364 (95.04) 18 (4.96) |
1 1.34 (0.64-2.82) |
0.43 |
|
History of physical, mental, and psychological stress in pregnancy No Yes |
106 (58.24) 76 (41.76) |
337 (92.58) 27 (7.42) |
1 8.52 (4.95-14.67) |
0.000 |
|
RH Rh+ Rh- |
174 (95.6) 8 (4.40) |
335 (92.03) 29 (7.97) |
1 0.53 (0.24-1.19) |
0.12 |
|
Mother’s, BMI > 18.4 24.9-18.5 29.9-25 ≤ 30 |
13 (7.14) 69 (37.91) 64 (35.16) 36 (19.78) |
16 (4.41) 133 (36.64) 141 (38.84) 74 (20.11) |
1 0.63 (0.28-1.41) 0.55 (0.25-1.22) 0.59 (0.25-1.39) |
0.27 0.14 0.23 |
In the multivariate regression analysis, the variables that had been significant in the single-variable analysis stage were included in the multivariate model. At this stage, the variables with a significant level of 0.2 and less were introduced to the model. The results of this model analysis show that there is a significant relationship between maternal gestational age, history of illness, medication abuse during pregnancy, mental stress during pregnancy, and multiple birth with LBW in the case and control groups (P < 0.05) (Tab. III).
Tab. III.
Results of multivariate conditional logistic regression of newborns born in Kurdistan province (west of Iran).
| Adjusted OR (95% CI) | P-value | ||
|---|---|---|---|
| Gestational weeks | ≤ 37 | 1 | 0.0001 |
| > 37 | 6.94 (3.11-15.50) | ||
| Distance between pregnancy | ≤ 3 years | 1 | 0.54 |
| > 3 years | 1.40 (0.46-4.22) | ||
| Pregnancy with IVF | No | 1 | 0.12 |
| Yes | 15.35 (0.45-512.7) | ||
| Multiple birth | Singleton | 1 | 0.0001 |
| ≤ twain | 85.81 (5.74-128.08) | ||
| Mother’s disease history | No | 1 | 0.002 |
| Yes | 3.66 (1.79-7.46) | ||
| History of bleeding | No | 1 | 0.1 |
| Yes | 2.81 (0.80-9.75) | ||
| Longing | No | 1 | 0.82 |
| Yes | 0.91 (0.39-2.08) | ||
| Smoking | No | 1 | 0.27 |
| Yes | 5.32 (0.13-205.80) | ||
| Exposed to second smoke | No | 1 | 0.2 |
| Yes | 1.69 (0.74-3.83) | ||
| Drug abuse | No | 1 | 0.006 |
| Yes | 2.23 (17.31-134.16) | ||
| Drug use under medical supervision | No | 1 | 0.39 |
| Yes | 1.48 (0.59-3.68) | ||
| History of physical, mental, and psychological stress in pregnancy | No | 1 | 0.0001 |
| Yes | 6.59 (2.52-17.18) |
Discussion
The prevalence of LBW is one of the most important health indicators and an indicator of the survival of the baby at the moment of birth. By recognizing the risk factors associated with LBW, it is possible to prevent LBW very significantly in newborns [1]. The results of this study show that there is a statistically significant relationship between LBW and maternal gestational age, mother’s disease history, medication abuse during pregnancy, psychological stress during pregnancy, and multiple pregnancies in the case and control groups (P < 0.05).
In Iran, the birth of LBW infants is a major cause of neonatal mortality (IMR). LBW has a direct relation with the duration of pregnancy: When the number of weeks of pregnancy is less than normal, the birth weight of the baby will be less than the normal weight due to insufficient growth of the foetus [15]
According to our study, premature infants (less than 37 weeks) had lower birth weight. The results of this study are consistent with the studies of Feresu et al. [19], Badshah et al. [20], and Muchemi et al.[21].
The results of various studies show that prematurity in developed countries and Iran is a common cause of LBW [15]. It is possible to detect different causes associated with underweight newborns – such as maternal diseases, genetic problems, and midwifery problems – to prevent the birth of a LBW baby. Moreover, the prevention measures also include teaching health promotion behaviors, pregnancy care, vaccination of pregnant women, proper education of health behaviors in fertility, improving economic, cultural, and social conditions, avoiding risky behaviors like smoking; in fact, with these measures, infants could be born with ideal weight [22]. In developing countries, this is one of the most important risk factors for birth weight and thus it is necessary to adopt methods to lessen this risk factor. Prenatal diseases and repeated infections in pregnancy are among the most effective factors relating to LBW [23, 24].
The results of this study show that mothers with a history of illness are three times more likely to have babies with LBW compared those who do not have a history of illness.
Rubari et al. in their stud represented that the prevalence of underweight in infants of mothers with a history of disease is higher than that of mothers without a history of disease [25].
Also, findings of Batist et al. study reported that mothers with a history of disease are more likely to have infants with low birth weight. These results were consistent with the results of our study [26].
However,, these results are not consistent with the research of Sharma et al. in Nepal [27] as well as the studies of Feresu et al. [19] and Badshah et al.[20] which declare that there is no significant relationship between the history of mothers and the birth of children with LBW in the case and control groups. The need to pay attention to pregnant mothers with other illnesses should be taken more into consideration.
Pregnancy bleeding can be caused by pregnancy diseases including vaginal infections, chlamydia, gonorrhea, and swelling and inflammation of the uterus. In the second and third trimesters of pregnancy, bleeding or spotting can indicate a dangerous condition, such as sudden detachment of the foetus from the uterus, that is likely to cause abortion or preterm delivery leading to premature infants [28, 29]. Which might be a reason for their LBW. The results of this study show that the history of bleeding points to the chance of having an underweight baby more than two times, which is statistically significant. These results are consistent with the results of Moradi et al. [2] and Eshraghian et al. [30] in Iran which found the relationship between bleeding during pregnancy and LBW risk in the case and control groups to be statistically significant. This risk factor in mothers should be taken into consideration in developing countries and the intervention design to reduce this risk factor.
Drugs used during pregnancy can affect the foetus. In fact, they may affect maternal and fetal health in the coming years or cause trichoderma to the foetus. The placenta allows the passage of many medications and dietary items. Pregnant women should be trained in other non-pharmacological methods to cope with stress, pain, and discomfort as well as other illnesses, and drug must be used only when it is necessary [22, 31, 32].
According to the results of the present study, arbitrary drug use increases the probability of giving birth to an underweight infant.
This relationship is in line with the results of Huang et al. [32]. Public health and maternity care programs should pay close attention to this risk factor.
Violence during pregnancy affects the birth weight of newborns. Since it can physically and mentally affect pregnant women, widespread planning is essential to reduce violence, especially physical violence, and convince the families about its subsequent consequences [19, 33]. Violence and mental stress during pregnancy can affect the birth weight of newborns. According to the results of our study, mental stress is strongly associated with LBW infants and increases the chance of having a LBW about six times this relationship is statistically significant. The results of this study are consistent with the results of Ansari et al. [29, 34].
The studies of Johnson et al. in Canada and Kedy et al. in Uganda to investigate the relationship between mental stress and adverse outcomes of pregnancy show that there is a significant relationship between LBW and mental stress during pregnancy. These results are consistent with the results of our study [35, 36]. The results of Leung et al. do not show a statistically significant relationship between violence during pregnancy and low risk of LBW [37]. Thus, violence is a risk factor that affects women in a way that needs to be addressed and included in care plans. Since most violence comes from husbands and takes place in neighborhoods where people of low socioeconomic status live, it seems important to arrange awareness classes at times and places suitable for husbands. To reduce this risk factor, interventions targeting pregnant women should be considered.
The restriction of intrauterine growth is three times more common in twin pregnancies than in single pregnancies, the limitation of intrauterine growth is asymmetric in multiform pregnancies. The relative immaturity in placenta and competition of twins on nutrients are the most likely causes of LBW [27]. The results of this study show that twin pregnancies increase the chance of LBW infants. These results are consistent with the results of Ansari et al. [34]. In other studies, it has been shown that a twin pregnancy is somewhat related to LBW for newborns [38, 39]. Multiple pregnancies in developing countries and in societies like Iran require more care during pregnancy and delivery.
This study was conducted in rural areas of one of the Iranian provinces and may have limitations in terms of generalizing its results in respect to the whole country. Therefore, it is suggested that given the presence of potential groups in countries like Iran, researchers would have to place the nesting control case nationally on their own agendas.
Conclusions
Birth weight in infants depends on several causes, not a single cause. Drug Abuse, the interval between pregnancies, and the history of bleeding are among the most preventable factors associated with LBW. In addition, other risk factors during pregnancy should be identified and nullified to reduce the number of LBW babies.
Figures and tables
Acknowledgements
Funding sources: this research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
The present study is extracted from an MA thesis of the department of epidemiology and biostatics with a code of ethics IR.MUK.RIC 1395.4, Kurdistan University of Medical Sciences. We thank the deputy of health of Kurdistan University of Medical Sciences and the staff of all the provincial health centers who helped us with this research.
Footnotes
Conflicts of interest statement
The authors declare no conflict of interest.
Authors’ contributions
This study was done by ZKH and GM participated in the design of the study. Data collection was done by ZKH and MZ. ZKH, GM and EG performed the statistical analysis. ZKH, GM and EG performed the coordination and helped with the drafting of the manuscript. The authors read and approved the final version of the manuscript.
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