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. 2022 Feb 18;34(6):e23732. doi: 10.1002/ajhb.23732

Development of birthweight and length for gestational age and sex references in Yucatan, Mexico

Hugo Azcorra 1,✉, Federico Dickinson 2, Nina Mendez‐Dominguez 3, Rebekka Mumm 4, Graciela Valentín 2
PMCID: PMC9285606  PMID: 35179265

Abstract

Objective

To develop sex‐ and gestational age specific reference percentiles and curves for birth weight and length for Yucatec neonates using data from birth registers of infants born during 2015–2019.

Material and methods

Observational, descriptive, epidemiologic study in a 5‐year period including every registered birth in the state of Yucatan, Mexico using birth registries. A total of 158 432 live, physically healthy singletons (76 442 females and 81 990 males) between 25 and 42 weeks of gestation were included in the analysis. We used the LMS method to construct smoothed reference centiles (3rd, 10th, 25th, 50th, 75th, 95th, and 97th) and curves for males and females separately.

Results

Mean maternal age was 26 (SD = 6.22) years. Fifty‐two percent of births occurred by vaginal delivery, 37% were firstborn and similar proportions were second (33%) and third or more (30%) born. 5.5% of newborns included in the references corresponds to neonates born before 37 weeks of gestation (5.9% boys and 5.1% girls). In both sexes, the percentage of infants with a birthweight less than 2500 g was 6.7%. The birthweight at the 50th percentile for males and females at 40 weeks of gestation in this cohort was 3256 and 3167 g, respectively, and the corresponding values for birth length were 50.23 and 49.84 cm (mean differences between sexes: 89 g and 0.40 cm, respectively).

Conclusion

The reference percentile and curves developed in this study are useful for research purposes and can help health practitioners to assess the biological status of infants born in Yucatán.

1. INTRODUCTION

From a human biology perspective, size at birth (weight and length) represents a crude indicator of intrauterine growth, resulting from the complex interaction between three main components: (1) parental and offspring's genetic load, (2) the mother's own pre‐ and postnatal growth and her nutritional status before conception and during pregnancy, and (3) socioenvironmental exposures experienced by the mother during pregnancy. Recent evolutionary models propose that phenotype at birth is also shaped by nutritional histories of recent ancestors, particularly through maternal line (Kuzawa, 2005; Kuzawa, 2008; Wells, 2003; Wells, 2007).

From an epidemiological point of view, impaired fetal growth is linked to higher neonatal mortality (Katz et al., 2013; Lawn, et al., 2005) and may affect cognitive development during childhood (de Bie et al., 2010). The theoretical frame of developmental programming suggests that extreme neonatal birthweight (small or large) is associated with a set of structural and functional anomalies that predispose individuals to cardiovascular and metabolic diseases at different stages of postnatal life (Gluckman et al., 2008; Thornburg, 2015). Moreover, lower birthweight may be correlated with a decreased likelihood of attainment of higher education and a higher risk of lower employment rate and earnings in adulthood (Bilgin et al., 2018; Lambris et al., 2021).

Given the importance of fetal growth, weight and length at birth are assessed at birth to serve as reference when planning appropriate, timely interventions. Neonatal evaluations are usually done by comparing the infants with gestational age and sex‐specific reference or standards based on newborns' weight or estimated fetal weights. While standards reflect the ideal or desirable growth pattern, reference charts describe the growth pattern of infants who were selected as the source sample, and their ability to discriminate between normal and abnormal growth is limited. Generally, reference values of size at birth are developed from observations of a single or few human groups, and even if translated to diverse populations, references seldom reflect growth of all populations. Therefore, low‐ and middle‐income countries' (LMICs) populations may be underrepresented and considered as of secondary importance.

In the last decade, several international standards for fetal growth and birthweight have been constructed (e.g. Fenton & Kim, 2013; Kiserud et al., 2017; Mikolajczyk et al., 2011; Villar et al., 2014). These standards assume that, in absence of any physiological or environmental constraint, the pattern of fetal growth is similar across ethnic groups. However, when the pattern of different countries is compared with these global standards, significant differences in fetal growth and birthweight, independently of maternal phenotype and fetal sex, have been shown (Kiserud et al., 2017). Moreover, several studies have shown a wide variation in the birthweight of newborns depending on ethnicity or ancestry (Ratnasiri et al., 2018; Spada et al., 2018; Zhang & Bowes Jr., 1995). Overall, these findings highlight the effect of plasticity, the importance of normal variability during prenatal growth and the significance of developing population‐specific references. The recent availability of data on birthweight, length and gestational age collected routinely in hospitals and compiled by the Birth Registration Systems in LMICs, provides the opportunity for researchers to produce population‐specific references and growth curves for regions with particular demographic characteristics and epidemiological profiles. The development of references for birth weight and length are particularly important in populations where socioeconomic disparities among individuals, reflected in maternal phenotype, risk behavior and access to quality health services, impact fetal growth.

The present study took place in the Mexican state of Yucatan, one of the poorer states of the country, which is located in southeast Mexico. By 2020, Yucatan had a population of about 2.3 million people distributed among 106 municipalities including Merida, which is the state capital. More than 40% of the total population of Yucatan resides in Merida (INEGI, 2020). Maya people, the largest indigenous group in America, reside in rural communities and cities of Yucatan. The Maya as a human group is represented by underserved people living in adverse socioeconomic conditions in terms of income and access to quality education and health services compared to non‐Maya people. Yucatan is also characterized by a high prevalence of cardiometabolic diseases among the adult population (ENSANUT, 2018) and the coexistence of low height‐for‐age and excess body weight in children (double burden of malnutrition) (Mendez et al., 2015; Varela‐Silva et al., 2012). Native population from the Yucatan Peninsula exhibit specific genetic characteristics explained by a long history of geographic and cultural isolation from the rest of the country. The patterns of ancestry are still present in mestizo individuals inhabiting rural and urban sites from Yucatan (Moreno‐Estrada et al., 2014). These characteristics could be translated to particular growth patterns in individuals from this population, which underlies the need for population‐specific references of weight and length at birth. As has been recently discussed by Thompson (2021), given the selection of samples representative of broad regions (mainly from north and South America, Europe and Africa) and the use of rigid inclusion criteria during the recruitment of participants, international standards likely do not capture the current genetic, social and cultural variability of our species.

In the present study, we present sex specific reference percentiles for weight and length at birth at different gestational ages for Yucatec neonates using birth registries of infants born between 2015 and 2019. Along with this purpose, this study describes how neonates' weight and length from this Mexican region vary according to weeks of gestation and sex.

2. METHODS

The present manuscript depicts an observational, descriptive, epidemiologic study including every registered birth in the state of Yucatan, Mexico, over a 5‐year period to provide a reference of size at birth and characterize Yucatec newborns according to maternal and pregnancy related characteristics of the newborns. The reference percentiles and curves we constructed were based on all births contained in the datasets from the Subsystem of Birth Registration (SINAC) of Mexico. The SINAC includes the compilation, storage, and validation of data collected in every birth within the national territory. In Yucatan, the system compiles information from more than 100 public and private hospitals distributed across the state. For the purposes of the reference construction, we used databases corresponding to all live births occurring in Yucatan between January 1, 2015, and December 31, 2019.

We defined the variables of interest for reference construction and descriptive purposes and transformed data from birth registries into variables regarding (a) newborns, (b) maternal characteristics, and (c) pregnancy. Neonatal sex, gestational age, weight, and length were used to produce reference percentiles and curves. Maternal sociodemographic characteristics, including age, level of education, ethnicity and marital status of the studied population were recorded for descriptive purposes. Birth type and order were also recorded as descriptive variables.

Perinatal and maternal sociodemographic data were obtained by neonatologists and/or gynecologist and trained nursing staff, respectively. Gestational length was calculated according to the last menstrual date and defined as a categorical variable indicating completed weeks of gestation. Birth weight and length were obtained within the first hour after birth and recorded in grams and centimeters, respectively, following the ruling protocols for neonatal assessment. These protocols are mandatory for all health personnel nationwide (NOM‐008‐SSA2‐1993, 1994) and continuous education is provided in this regard by every health institution (NOM‐005‐SSA3‐2010, 2011). Neonates' weight was recorded with the bare infant lying in the weighing tray of an electronic scale (Seca, model 354©, 10 g of precision). Recumbent length was measured using a pediatric infantometer (Seca, model 210©) to the nearest centimeter. The same anthropometric equipment models were used in all hospitals in Yucatan, following the specifications for health infrastructure. According to their type of birth and birth order, infants were grouped in (1) born by vaginal delivery and (2) born through cesarean section, and (1) first, (2) second, (3) third and more, respectively. The age of mothers was used as a numerical variable in years and then grouped into (1) <20, (2) 20–29, (3) 30–39, and (4) ≥40. Maternal education was categorized as: (1) low: none, primary school and junior high school; (2) medium: high school; and (3) high: university. The use of language was used as a proxy for Maya ancestry/ethnicity (Colantonio et al., 2003; Relethford, 1995). According to their marital status, women were grouped in (1) with partner and (2) without partner at infants' birth.

The original datasets include information on 178 588 infants born between 2015 and 2019. We exclude infants from multiple pregnancies (n = 3004, 1.7%), infants with syndromic and congenital anomalies affecting in utero growth (n = 4708, 2.7%) and infants whose mothers do not reside in Yucatan (n = 6422, 3.6%). We used weight/gestational age, length/age and weight/length plots to identify cases with birth weights and lengths falling within an infeasible range for each gestational age and then excluded these data from the analysis (n = 508, 0.3%). We restricted our analysis to infants born between weeks 25 and 42 of gestation. The final sample used to construct the centile references and curves consisted of 163 946 singletons (81 973 female and 81 973 male).

2.1. Centile modeling

We developed reference centiles for birthweight and length using the LMS (skewness, median, and coefficient of variation, respectively) method for smoothing reference centile curves (Cole, 1990; Cole & Green, 1992) and its extension (Generalized Additive Models for Location, Scale and Shape [GAMLSS]) by Rigby and Stasinopoulos (2004, 2006). We used the statistical software R (R Development Core Team, 2011). We used algorithms provided in the R package ‘gamlss’ to choose optimal degrees of freedoms for the parameters L, M, and S. For details see Stasinopoulos et al. (2017).

2.2. Ethical concerns

Retrospective, secondary, anonymized datasets were used under permission from the Ministry of Health of the State of Yucatan for the purpose of the present study. The health authorities exempted this research from ethical review because involved non‐identifiable data and datasets are in the public domain.

3. RESULTS

On average, maternal age was 26 (SD = 6.22) years, 18% were younger than 20 years old and 2% of them were aged over 40 (Table 1). Sixty‐five percent of women had a low educational level (none, primary and junior high school) and 15% finished university or any technical course. Most (92%) mothers had a partner at the time of their offspring's birth and 18% can be considered as Maya based on their use of the Maya language. A very low proportion of mothers (1%) did not receive prenatal attention and around 80% attended the first prenatal consultation during first trimester of pregnancy. Fifty‐two percent of infants were born by vaginal delivery, 38% were firstborns and similar proportions were born at least in second (33%) or third order (30%). 5.5% of newborns included in the references were born earlier than 37 weeks of gestation (5.9% boys and 5.1% girls); in both sexes the percentage of infants with a birthweight less than 2500 g was 6.7%.

TABLE 1.

Sociodemographic and pregnancy characteristics of participant women

Characteristic Frequency (%)
Mothers
Age (years), mean (standard deviation) 26.22 (SD = 6.22)
Age categories
<20 27 857 (18%)
20–29 86 801 (55%)
30–39 40 807 (26%)
≥40 2927 (2%)
Levels of education attained
Low: None, primary and junior high school 98 182 (65%)
Medium: High school 30 071 (20%)
High: University and more 23 675 (15%)
Use of Maya language
No 118 261 (82%)
Yes 25 297 (18%)
Marital status
Without partner 11 572 (8%)
With partner 138 694 (92%)
Receive prenatal attention
No 1787 (1%)
Yes 152 281 (99%)
Trimester of pregnancy when attended to the first prenatal consultation
First 120 646 (79%)
Second 29 469 (19%)
Third 3495 (2%)
Number of prenatal consultations
1–5 37 232 (24%)
≥ 6 115 049 (76%)
Newborns
Sex
Females 81 973 (48%)
Males 81 973 (52%)
Birth order (%)
First 58 820 (37%)
Second 51 345 (33%)
Third or more 47 776 (30%)
Type of birth (%)
Vaginal delivery 82 103 (52%)
Cesarean section 74 414 (48%)
Apgar
Low (0–3 points) 613 (0.5%)
Medium (4–6 points) 769 (0.5%)
High (7–10 points) 156 646 (99%)
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Smoothed percentiles (3rd, 10th, 25th, 50th, 75th, 90th, and 97th) for birthweight and length of male and female newborns for 25–42 weeks of gestational age are presented in Tables 2, 3, 4, 5. The median (50th percentile) weight for males and females at 40 weeks of gestation in this cohort is 3255 and 3167 g, respectively, and the corresponding values for birth length were 50.23 and 49.84 cm (mean intersexual differences: 88 g and 0.40 cm, respectively). Male infants were heavier and longer than females across all gestational ages but the intersexual differences for birthweight were greater for the weeks 30–31 and 38–42 of gestation. For length, differences were greater for weeks 25–31 and 38–42. Figures 1, 2, 3, 4 show reference charts for male and female newborns based on the smoothed‐percentile values. The number of newborns for each gestational week together with their mean values and standard deviations are included in Supplementary materials.

TABLE 2.

Smoothed birthweight percentiles (P) for Yucatecan newborns (boys) during 2015–2019

Gestation (weeks) P3 P10 P25 P50 P75 P90 P97 M L S
25 503 562 629 712 805 898 1000 712 0.064 0.183
25.5 517 582 654 744 846 947 1058 744 0.101 0.190
26 532 602 681 779 889 1000 1120 779 0.137 0.198
26.5 549 625 711 818 938 1057 1188 818 0.168 0.205
27 567 650 743 859 989 1120 1261 859 0.198 0.212
27.5 587 676 777 903 1044 1185 1338 903 0.222 0.218
28 608 705 814 950 1102 1254 1419 950 0.246 0.224
28.5 633 737 854 1000 1163 1325 1502 1000 0.263 0.228
29 662 773 898 1053 1227 1400 1588 1053 0.279 0.231
29.5 696 814 946 1111 1296 1479 1678 1111 0.290 0.232
30 736 860 1001 1175 1369 1562 1771 1175 0.301 0.232
30.5 783 914 1062 1244 1447 1649 1866 1244 0.306 0.228
31 840 978 1132 1322 1533 1741 1966 1322 0.311 0.225
31.5 908 1051 1212 1408 1626 1841 2071 1408 0.312 0.217
32 988 1136 1302 1505 1728 1948 2183 1505 0.312 0.210
32.5 1078 1232 1403 1611 1839 2063 2301 1611 0.310 0.200
33 1180 1339 1514 1726 1959 2186 2427 1726 0.308 0.191
33.5 1292 1456 1635 1852 2089 2318 2562 1852 0.313 0.181
34 1412 1581 1765 1986 2226 2458 2702 1986 0.318 0.172
34.5 1537 1711 1900 2126 2369 2603 2848 2126 0.345 0.164
35 1664 1845 2041 2273 2521 2758 3005 2273 0.372 0.156
35.5 1792 1982 2185 2425 2680 2923 3174 2425 0.412 0.152
36 1920 2118 2330 2578 2841 3089 3345 2578 0.452 0.147
36.5 2047 2252 2470 2725 2992 3243 3501 2725 0.490 0.141
37 2175 2384 2604 2859 3126 3375 3630 2859 0.528 0.135
37.5 2302 2509 2726 2976 3237 3480 3727 2976 0.539 0.128
38 2410 2612 2825 3069 3323 3559 3799 3069 0.549 0.120
38.5 2484 2683 2893 3134 3384 3616 3852 3134 0.534 0.117
39 2535 2733 2941 3180 3428 3659 3894 3180 0.519 0.114
39.5 2579 2775 2981 3219 3466 3695 3929 3219 0.511 0.112
40 2616 2812 3018 3255 3501 3730 3963 3255 0.503 0.110
40.5 2641 2840 3048 3288 3536 3766 4000 3288 0.539 0.111
41 2651 2856 3071 3317 3570 3806 4044 3317 0.575 0.112
41.5 2642 2858 3082 3340 3605 3849 4097 3340 0.625 0.117
42 2620 2849 3088 3360 3639 3896 4156 3360 0.676 0.122
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Note: L, M, and S parameters from the LMS method (Cole, 1990; Cole & Green, 1992) for skewness (L), median (M), and coefficient of variation (S).

TABLE 3.

Smoothed birthweight percentiles (P) for Yucatecan newborns (girls) during 2015–2019

Gestation (weeks) P3 P10 P25 P50 P75 P90 P97 M L S
25 493 553 618 693 772 847 923 693 0.552 0.165
25.5 516 580 647 727 811 889 971 727 0.537 0.167
26 539 606 678 762 850 934 1020 762 0.522 0.168
26.5 563 633 709 798 892 981 1073 798 0.508 0.170
27 586 660 740 835 935 1030 1129 835 0.493 0.173
27.5 608 687 772 873 980 1082 1187 873 0.478 0.177
28 631 715 805 912 1027 1136 1250 912 0.463 0.180
28.5 655 744 840 955 1077 1195 1318 955 0.448 0.184
29 681 775 878 1001 1133 1260 1393 1001 0.433 0.189
29.5 710 811 921 1054 1196 1334 1478 1054 0.418 0.193
30 744 852 971 1114 1268 1418 1575 1114 0.403 0.198
30.5 785 901 1028 1183 1350 1513 1685 1183 0.388 0.201
31 833 958 1096 1262 1444 1621 1809 1262 0.373 0.205
31.5 892 1026 1174 1355 1552 1744 1948 1355 0.359 0.205
32 962 1106 1265 1459 1671 1879 2099 1459 0.344 0.206
32.5 1045 1198 1367 1574 1800 2022 2258 1574 0.329 0.203
33 1140 1302 1481 1699 1937 2171 2420 1699 0.314 0.199
33.5 1249 1418 1605 1832 2080 2323 2582 1832 0.299 0.191
34 1370 1544 1737 1970 2226 2475 2740 1970 0.284 0.184
34.5 1498 1677 1873 2111 2370 2623 2891 2111 0.269 0.174
35 1631 1813 2012 2253 2514 2767 3036 2253 0.254 0.165
35.5 1768 1952 2153 2395 2656 2910 3178 2395 0.239 0.156
36 1906 2091 2293 2535 2796 3049 3315 2535 0.224 0.147
36.5 2043 2229 2431 2672 2931 3181 3444 2672 0.210 0.139
37 2176 2361 2561 2799 3055 3300 3558 2799 0.195 0.131
37.5 2297 2479 2676 2909 3159 3399 3650 2909 0.180 0.124
38 2395 2575 2767 2995 3239 3472 3716 2995 0.165 0.117
38.5 2465 2641 2830 3054 3293 3521 3759 3054 0.150 0.113
39 2513 2687 2874 3096 3331 3557 3792 3096 0.135 0.109
39.5 2550 2724 2911 3132 3367 3592 3827 3132 0.120 0.108
40 2583 2758 2945 3167 3403 3629 3865 3167 0.105 0.107
40.5 2614 2790 2978 3201 3439 3666 3905 3201 0.090 0.107
41 2633 2811 3002 3228 3470 3703 3946 3228 0.075 0.108
41.5 2630 2813 3010 3244 3495 3736 3990 3244 0.061 0.112
42 2614 2803 3008 3252 3515 3769 4036 3252 0.046 0.115
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Note: L, M, and S parameters from the LMS method (Cole, 1990; Cole & Green, 1992) for skewness (L), median (M), and coefficient of variation (S).

TABLE 4.

Smoothed birth length percentiles (P) for Yucatecan newborns (boys) during 2015–2019

Gestation (weeks) P3 P10 P25 P50 P75 P90 P97 M L S
25 28.10 29.27 30.51 31.95 33.46 34.88 36.34 31.95 −0.014 0.068
25.5 28.30 29.56 30.88 32.41 34.01 35.51 37.04 32.41 0.144 0.071
26 28.51 29.86 31.28 32.90 34.59 36.16 37.75 32.90 0.303 0.075
26.5 28.74 30.19 31.70 33.42 35.20 36.83 38.49 33.42 0.457 0.077
27 29.00 30.55 32.15 33.97 35.82 37.53 39.24 33.97 0.612 0.080
27.5 29.27 30.93 32.62 34.53 36.46 38.23 39.98 34.53 0.761 0.082
28 29.58 31.33 33.12 35.11 37.11 38.93 40.72 35.11 0.911 0.084
28.5 29.92 31.77 33.64 35.71 37.77 39.62 41.45 35.71 1.056 0.085
29 30.31 32.25 34.20 36.33 38.44 40.32 42.15 36.33 1.202 0.087
29.5 30.76 32.79 34.79 36.97 39.11 41.00 42.84 36.97 1.346 0.086
30 31.30 33.38 35.43 37.64 39.79 41.68 43.50 37.64 1.490 0.086
30.5 31.92 34.05 36.13 38.35 40.49 42.35 44.15 38.35 1.635 0.084
31 32.65 34.80 36.88 39.09 41.20 43.04 44.79 39.09 1.781 0.082
31.5 33.48 35.63 37.70 39.88 41.95 43.74 45.44 39.88 1.928 0.079
32 34.41 36.54 38.57 40.71 42.73 44.46 46.10 40.71 2.075 0.075
32.5 35.41 37.51 39.49 41.57 43.52 45.19 46.77 41.57 2.221 0.072
33 36.48 38.52 40.44 42.45 44.32 45.93 47.44 42.45 2.367 0.068
33.5 37.60 39.56 41.41 43.34 45.14 46.67 48.11 43.34 2.507 0.064
34 38.73 40.61 42.38 44.21 45.93 47.39 48.76 44.21 2.648 0.059
34.5 39.85 41.64 43.32 45.06 46.68 48.07 49.37 45.06 2.775 0.055
35 40.94 42.63 44.22 45.87 47.41 48.73 49.96 45.87 2.903 0.051
35.5 41.98 43.59 45.10 46.66 48.13 49.38 50.55 46.66 2.996 0.048
36 42.95 44.48 45.92 47.41 48.82 50.01 51.14 47.41 3.090 0.045
36.5 43.83 45.28 46.66 48.10 49.44 50.59 51.68 48.10 3.121 0.043
37 44.59 45.98 47.30 48.68 49.98 51.10 52.14 48.68 3.152 0.041
37.5 45.22 46.55 47.82 49.15 50.41 51.49 52.52 49.15 3.084 0.039
38 45.70 46.98 48.20 49.50 50.73 51.79 52.79 49.50 3.015 0.038
38.5 46.03 47.27 48.46 49.73 50.94 51.98 52.97 49.73 2.857 0.037
39 46.29 47.49 48.66 49.90 51.09 52.12 53.11 49.90 2.699 0.036
39.5 46.55 47.72 48.85 50.06 51.24 52.26 53.23 50.06 2.538 0.036
40 46.76 47.90 49.02 50.23 51.39 52.41 53.39 50.23 2.377 0.035
40.5 46.86 48.02 49.15 50.38 51.56 52.60 53.61 50.38 2.258 0.036
41 46.89 48.07 49.24 50.51 51.74 52.81 53.85 50.51 2.139 0.037
41.5 46.87 48.10 49.32 50.63 51.91 53.03 54.11 50.63 2.036 0.038
42 46.84 48.12 49.38 50.74 52.08 53.25 54.38 50.74 1.934 0.039
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Note: L, M, and S parameters from the LMS method (Cole, 1990; Cole & Green, 1992) for skewness (L), median (M), and coefficient of variation (S).

TABLE 5.

Smoothed birth length percentiles (P) for Yucatecan newborns (girls) during 2015–2019

Gestation (weeks) P3 P10 P25 P50 P75 P90 P97 M L S
25 27.88 29.01 30.18 31.50 32.85 34.09 35.33 31.50 0.530 0.063
25.5 28.18 29.36 30.58 31.95 33.34 34.61 35.88 31.95 0.649 0.064
26 28.48 29.72 30.99 32.41 33.85 35.16 36.46 32.41 0.768 0.066
26.5 28.78 30.08 31.41 32.90 34.39 35.73 37.07 32.90 0.888 0.067
27 29.09 30.46 31.85 33.40 34.94 36.33 37.71 33.40 1.008 0.069
27.5 29.40 30.85 32.31 33.92 35.53 36.96 38.37 33.92 1.128 0.070
28 29.73 31.26 32.79 34.47 36.13 37.61 39.06 34.47 1.249 0.072
28.5 30.08 31.69 33.30 35.05 36.77 38.30 39.78 35.05 1.369 0.073
29 30.46 32.16 33.85 35.67 37.45 39.02 40.53 35.67 1.490 0.075
29.5 30.88 32.68 34.44 36.33 38.16 39.77 41.32 36.33 1.610 0.076
30 31.37 33.25 35.08 37.04 38.92 40.56 42.13 37.04 1.730 0.077
30.5 31.93 33.89 35.79 37.80 39.72 41.38 42.97 37.80 1.848 0.077
31 32.60 34.62 36.56 38.61 40.56 42.23 43.83 38.61 1.966 0.077
31.5 33.36 35.44 37.41 39.48 41.44 43.12 44.72 39.48 2.078 0.075
32 34.24 36.34 38.33 40.40 42.36 44.03 45.61 40.40 2.191 0.074
32.5 35.22 37.32 39.30 41.35 43.28 44.93 46.49 41.35 2.294 0.071
33 36.29 38.36 40.30 42.31 44.20 45.81 47.33 42.31 2.398 0.068
33.5 37.42 39.42 41.31 43.26 45.10 46.65 48.12 43.26 2.490 0.065
34 38.56 40.48 42.29 44.17 45.93 47.42 48.83 44.17 2.581 0.061
34.5 39.65 41.48 43.20 44.99 46.67 48.10 49.44 44.99 2.661 0.057
35 40.71 42.43 44.05 45.75 47.34 48.69 49.97 45.75 2.741 0.053
35.5 41.72 43.33 44.86 46.46 47.96 49.24 50.45 46.46 2.801 0.049
36 42.69 44.19 45.63 47.13 48.55 49.77 50.91 47.13 2.862 0.046
36.5 43.61 45.02 46.37 47.79 49.13 50.28 51.37 47.79 2.893 0.043
37 44.41 45.75 47.03 48.37 49.65 50.75 51.79 48.37 2.924 0.040
37.5 45.05 46.32 47.55 48.84 50.07 51.13 52.14 48.84 2.911 0.039
38 45.50 46.73 47.92 49.18 50.38 51.41 52.39 49.18 2.898 0.037
38.5 45.80 47.00 48.16 49.39 50.57 51.58 52.55 49.39 2.835 0.036
39 46.03 47.20 48.33 49.54 50.70 51.70 52.66 49.54 2.773 0.035
39.5 46.24 47.39 48.50 49.69 50.84 51.83 52.78 49.69 2.676 0.035
40 46.41 47.55 48.66 49.84 50.98 51.98 52.93 49.84 2.579 0.035
40.5 46.51 47.65 48.77 49.97 51.13 52.14 53.11 49.97 2.476 0.035
41 46.49 47.67 48.82 50.07 51.26 52.31 53.32 50.07 2.372 0.036
41.5 46.37 47.60 48.82 50.13 51.39 52.50 53.56 50.13 2.273 0.038
42 46.17 47.48 48.78 50.17 51.52 52.69 53.83 50.17 2.174 0.040
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Note: L, M, and S parameters from the LMS method (Cole, 1990; Cole & Green, 1992) for skewness (L), median (M), and coefficient of variation (S).

FIGURE 1.

FIGURE 1

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Centile curves for birthweight of Yucatecan newborns (girls) during 2015–2019, from week 25 to 42 of gestation

FIGURE 2.

FIGURE 2

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Centile curves for birthweight of Yucatecan newborns (boys) during 2015–2019, from week 25 to 42 of gestation

FIGURE 3.

FIGURE 3

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Centile curves for birth length of Yucatecan newborns (girls) during 2015–2019, from week 25 to 42 of gestation

FIGURE 4.

FIGURE 4

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Centile curves for birth length of Yucatecan newborns (boys) during 2015–2019, from week 25 to 42 of gestation

Complementarily, the LMS parameters of birthweight and length were used to develop a tool to calculate the individual z‐score and/or percentile of a newborn based on the presented references. Please see Resource Availability Statement to get access to the tool.

4. DISCUSSION

We produce sex‐ and gestational age specific reference percentiles and curves for birthweight and length for newborns from Yucatan, Mexico, using a large and population‐based dataset from birth registers of infants born during 2015–2019.

When comparing our cohort with references from other populations, including those residing in high income countries and LMICs, remarkable differences are identified in neonatal weight. Birthweight of Yucatec infants at 40 weeks of gestations is, on average, 470 g (males) and 408 g (females) lower than infants from Norway (Skjaerven et al., 2000), 358 g (males) and 303 g (females) lower than infants from Canada (Kramer et al., 2001), 114 g (males), 81 g (males) lower than Brazilian infants (Pedreira et al., 2011) and 190 g (the same difference for both sexes) higher than newborns from South India (Kumar et al., 2013). Very few studies have analyzed the birthweight for gestational age in Mexican infants (Flores Huerta & Martínez Salgado, 2012; Ríos et al., 2008). Compared to infants included in a study undertaken in 33 hospitals belonging to the Mexican Social Security Institute which included all births occurring from June 2000 to March 2002 (Flores Huerta & Martínez Salgado, 2012), newborns of our cohort are, on average, 150, and 100 g lighter than infants from the north and center of Mexico. The difference with neonates from the north of Mexico is similar to that reported by Ríos et al. (2008) in their study with infants from Chihuahua State. However, mean birthweight of Yucatec infants is comparable with birthweight of infants from the south of the country (see Figures S1 and S2 for comparisons with other Mexican populations from 35 to 42 weeks of gestation).

Differences in weight and length between neonates from our study and those reported elsewhere may be explained by a number of variables that influence prenatal growth trajectories, including genetic characteristics of populations, maternal phenotype and physiology, socioeconomic conditions, stress and physical work during pregnancy, physical environment (temperature and altitude), parental health habits and gestational length (e.g., Kramer, 1987; Mallia et al., 2017; Mélançon et al., 2020; Wells & Cole, 2002). In the case of the population from Yucatan, ethnicity and maternal height may have an important influence on neonates' birthweight. We have previously reported that birthweight of infants from Maya mothers are 63 g lighter than infants from non‐Maya mothers (3087 g [SD = 408] vs 3150 g [SD = 404]) (Azcorra et al., 2016) and infants from mothers in the shortest quartile of height (129–147 cm) had a birthweight of −0.43 standard deviations compared with infants from mothers in the highest quartile (156–180 cm) (3076 g [SD = 406] vs 3272 g [SD = 397]) (Azcorra & Méndez, 2018). The height of adult women in the state of Yucatan is comparable to heights found among women in Guatemala and the Philippines, populations with the lowest heights in the world (NCD Risk Factor Collaboration [NCDRisC], 2016). From an evolutionary point of view, and particularly from Life History Theory and Parent‐Offspring Conflict Theory perspectives, natural selection has operated through physiological mechanisms that allow mothers to deliver infants with optimal birthweights to maximize their lifetime reproductive success under extant ecological conditions (Bereczkei et al., 2000; Blurton Jones, 1978; Thomas et al., 2004; Trivers, 1974). Therefore, lower birth weights are expected in populations exposed to chronic adverse living conditions affecting maternal phenotype intergenerationally.

In the present study, the references we provide can be used for research and assessment purposes. Since the references we produced derive from a population‐based dataset over 5‐year period, these can be used by researchers to compare their samples and answer a wide range of research questions about variability in growth and phenotype at birth. Few studies have analyzed the birthweight in Yucatec population and its relationship with maternal characteristics, neonatal mortality, and body composition during childhood (Azcorra et al., 2016; Azcorra et al., 2021; Azcorra & Méndez, 2018; Osorno‐Covarrubias et al., 2002; Varela‐Silva et al., 2009). We hope to stimulate future studies aimed at analyzing the link between size at birth and phenotype, and functional characteristics and illnesses during childhood, adolescence, and adulthood by using these references as a methodological tool that allow situating any sample from Yucatan in the context of the population this belong. This aspect is particularly relevant in the context of the population of Yucatan since some of the chronic‐degenerative diseases which have a very high prevalence in the state of Yucatan, including overweight/obesity, type 2 diabetes mellitus, hypertension and other cardio‐metabolic disorders, have been shown to be related with certain growth trajectories and structural and functional alterations occurring during the intrauterine stage (Gluckman et al., 2008; Thornburg, 2015).

In the clinical context, these reference percentile and curves can help health practitioners, including pediatricians, neonatologists and nurses, to assess the health status of newborns. The current official Mexican regulation for the care of the newborn (NOM‐007‐SSA2‐1993, 1994) recommends the use of any of the references developed by Lubchenco and collaborators (Lubchenco et al., 1963; Lubchenco et al., 1966) or Jurado‐García et al. (1970) to evaluate the birthweight and length of Mexican neonates. The reference percentiles developed by Lubchenco and collaborators are based on the birthweight of 5635 white infants born at Colorado General Hospital in the USA between 1948 and 1961 and those developed by Jurado‐García et al. (1970) derived from a study in which the birthweight of 16 807 infants was obtained in hospitals of Mexico City during 1968–1970. We consider that these references are not appropriate for their current use in Yucatec newborns since their data were collected more than 50 years ago and because they came from populations that differ genetically and socioeconomically from the population from Yucatan.

Since the references we produced include both low‐ and high‐risk pregnancies and were not tested against morbidity and mortality during neonatal stage, we do not recommend that these references be used to define abnormal/pathological fetal growth. Small for gestational age (SGA) and large for gestational age (LGA) are categories commonly used in the clinical practice and defined by centiles (SGA: <10th percentile, LGA: >90th percentile). We recommend great caution when using the references we developed to define these categories. Our references certainly allow researchers to situate an individual in terms of their birthweight and length in the context of the population she or he belongs to. The integration of other indicators of fetal health, such as heart rate, biophysical profile (estimated fetal weight and abdominal circumference) and amniotic fluid volume, and placental function (all of them routinely assessed during pregnancy), in addition to size at birth, may enhance the assessment of intrauterine growth (Zhang, Merialdi, et al., 2010). The combination of antenatal assessment and birth weight may help to distinguish between normal and abnormal fetal growth. References for Yucatec newborns' weight and length may also in time serve as a baseline comparison for future interventions by public health bodies wishing to evaluate the wellbeing of mothers and their offspring.

The data we present in this article refer to offspring growth under particular environmental conditions at a specific point in time. Intergenerational changes in birthweight have been described in several populations (see for example Chike‐Obi et al., 1996; Kramer et al., 2002; Schack‐Nielsen et al., 2006). Intergenerational changes in birthweight can be driven by changes in length of gestation and fetal growth (Oken, 2013), which in turn, can be influenced by obstetric practices (Zhang, Joseph, & Kramer, 2010), maternal age and secular changes in maternal phenotype including height, weight and gestational weight gain. Thus, these references must be updated regularly to account for changes in these factors.

The limitations of the present study are inherent to methodological aspects, such as the retrospective nature of the study. As birth length is measured in complete gestational weeks as a discrete variable, the variability of length in full centimeters is limited. The calculation of references always requires smoothing which is more difficult for a discrete variable. By using different algorithms in the R package ‘gamlss’ e.g. the function lms() to choose the optimal degree of smoothing, we reduced these limitations (Stasinopoulos et al., 2017). We are aware that many health professionals participated in obtain infants' weight and length, however, we believe this situation have a minimal effect on the accuracy of the percentiles.

5. CONCLUSION

In this article we present reference percentiles and curves for birthweight and length based on a large and population‐based dataset from birth registers of infants born in Yucatan during 2015–2019. Percentiles show that birthweight of Yucatec neonates is lower than in populations with different levels of income and also with respect to populations belonging to countries with levels of economic development similar to Mexico. The references for Yucatec newborns regarding weight and length may not only be used for research in inter‐population comparisons but may also be used in the clinical context in conjunction with relevant antenatal fetal and maternal indicators to assess newborn's health. These references must be updated in the future to account for secular changes in newborns' size and for socioeconomic and political changes in the region.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

AUTHOR CONTRIBUTIONS

Hugo Azcorra: Data curation (equal); methodology (equal); writing – original draft (equal); writing – review and editing (equal). FEDERICO DICKINSON: Conceptualization (equal); methodology (equal); writing – review and editing (equal). Nina Mendez‐Dominguez: Data curation (equal); methodology (equal); writing – review and editing (equal). Rebekka Mumm: Data curation (equal); formal analysis (equal); methodology (equal); writing – review and editing (equal). Graciela Valentín: Data curation (equal); writing – review and editing (equal).

Supporting information

Table S1 Mean (standard deviation) of birthweight and length of Yucatec male infants born during 2015–2019 by week of gestation

Figure S1 Comparison of the 50th birthweight percentiles between Yucatec male neonates and neonates from the north, center and south of Mexico and Mexico City.

Figure S2 Comparison of the 50th birthweight percentiles between Yucatec female neonates and neonates from the north, center and south of Mexico and Mexico City.

Click here for additional data file. (340.5KB, docx)

ACKNOWLEDGMENTS

The authors thank the Subsystem of Births Information (SINAC) and the Ministry of Health of the State of Yucatan for providing us with access to the dataset.

Azcorra, H. , Dickinson, F. , Mendez‐Dominguez, N. , Mumm, R. , & Valentín, G. (2022). Development of birthweight and length for gestational age and sex references in Yucatan, Mexico. American Journal of Human Biology, 34(6), e23732. 10.1002/ajhb.23732

Contributor Information

Hugo Azcorra, Email: hugoazpe@hotmail.com.

Nina Mendez‐Dominguez, Email: ninamendezdominguez@gmail.com.

DATA AVAILABILITY STATEMENT

The developed tool to calculate the individual z‐score and/or percentile of a newborn based on the presented references are openly available in Researchgate profiles of the authors of this article.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Table S1 Mean (standard deviation) of birthweight and length of Yucatec male infants born during 2015–2019 by week of gestation

Figure S1 Comparison of the 50th birthweight percentiles between Yucatec male neonates and neonates from the north, center and south of Mexico and Mexico City.

Figure S2 Comparison of the 50th birthweight percentiles between Yucatec female neonates and neonates from the north, center and south of Mexico and Mexico City.

Click here for additional data file. (340.5KB, docx)

Data Availability Statement

The developed tool to calculate the individual z‐score and/or percentile of a newborn based on the presented references are openly available in Researchgate profiles of the authors of this article.

Articles from American Journal of Human Biology are provided here courtesy of Wiley

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