Introduction: Childhood obesity is a growing public health problem with long-term consequences. Understanding the early contributing factors is crucial for prevention and early intervention. This study explored the influence of breastfeeding, birth weight, gestational age, parental education, and sex on body mass index (BMI) during infancy. Methods: Standardized weight and height measurements of children followed a common World Health Organization protocol. Information on sex, gestational age, birth weight, breastfeeding practices and duration, family income, and mother’s educational level, as well as other sociodemographic factors, was collected from clinical records. Linear regression models were calculated. Results: This study analyzed factors associated with overweight and obesity in 286 children under 5 years of age, using data from daycare records. Several significant associations were found. Regarding breastfeeding, while 85% of children received breast milk, only 23% did so exclusively for at least six months. Although no significant difference was observed in BMI change between exclusive and partial breastfeeding groups between birth and 5 years of age, the duration of exclusive breastfeeding, the birth BMI, and the educational level predicted 54% of the variability in BMI percentile change from birth to two years (p = 0.001). In addition, girls showed significantly longer exclusive breastfeeding. Regarding gestational age, preterm infants showed a significantly greater increase in BMI percentile compared to term infants. Gestational age also proved to be a significant factor in explaining BMI variability up to 5 years of age. Regarding sex, at age 5, boys showed a significantly higher prevalence of overweight and obesity than girls. With respect to family income, no statistically significant difference was found in BMI change between birth and 2 years of age; however, this variable warrants further investigation in future studies with greater statistical power. Finally, birth BMI was a significant predictor of BMI variability at 5 years of age. Conclusions: In this study, gestational age, sex, birth BMI, and the duration of exclusive breastfeeding were the most important determinants of BMI and the prevalence of overweight and obesity in children up to 5 years of age. Further studies are needed to thoroughly explore the role of family income and other factors.

Background: Adolescents with a chronological age of less than 15 years or a gynecological age of less than 2 years may have a higher probability of complications because they are more likely to conceive within 1 to 2 years of menarche and, therefore, are still growing and maturing. This could impair their ability to adapt to the physiological demands of pregnancy. Objective: To evaluate the relationship between chronological age and gynecological age with low birth weight and small for gestational age among adolescent mothers in Mexico City. Methods: A retrospective cohort design of adolescent mother–child dyads was conducted. The study followed 1242 adolescents under 19 years of age and their children, collecting data on physical, socioeconomic, and clinical characteristics, including hemoglobin levels. Low birth weight was assessed using the Intergrowth-21st project standards and categorized as above or below 2500 g. The mothers were grouped by chronological age (<15 years and ≥15 years) and gynecological age (<3 years and ≥3 years). Adjusted odds ratios were calculated using binary logistic regression models. The outcome variables were low birth weight and small for gestational age. The independent variables included gynecological age, chronological age, age at menarche, hemoglobin concentration, and gestational weight gain, among others. All independent variables were converted to dummy variables for analysis. Calculations were adjusted for the following variables: marital status, maternal education, occupation, educational lag, family structure, socioeconomic level, pre-pregnancy body mass index, and initiation of prenatal care. Results: The average age of the participants was 15.7 ± 1 years. The frequency of small for gestational age and low birth weight was 20% and 15.3%, respectively. Factors associated with small for gestational age included gynecological age < 3 years [aOR = 2.462, CI 95%; 1.081–5.605 (p = 0.032)], hemoglobin < 11.5 g/dL [aOR = 2.164, CI 95%; 1.08–605 (p = 0.019)], insufficient gestational weight gain [aOR = 1.858, CI 95%; 1.059–3.260 (p = 0.031)], preterm birth [aOR = 1.689, CI 95%; 1.133–2.519 p = 0.01], and living more than 50 km from the care center [aOR = 2.256, CI 95%; 1.263–4.031 (p = 0.006)]. An early age of menarche [aOR = 0.367, CI 95%; 0.182–0.744 (p = 0.005)] showed a protective effect against small for gestational age. Factors associated with low birth weight included gynecological age < 3 years [aOR = 3.799, CI 95%; 1.458–9.725 (p = 0.006)], maternal age < 15 years [aOR = 5.740, CI 95%; 1.343–26.369 (p = 0.019)], preterm birth [aOR = 54.401, CI 95%; 33.887–87.335, p = 0.001], living more than 50 km from the care center [aOR = 1.930, CI 95%; 1.053–3.536 (p = 0.033)], and early age of menarche [aOR = 0.382, CI 95%; 0.173–0.841 (p = 0.017), which demonstrated a protective effect, respectively. Conclusions: The study concludes that biological immaturity, particularly early gynecological age, significantly contributes to adverse birth outcomes during adolescent pregnancies. Interestingly, early menarche appeared to have a protective effect, whereas chronological age was not a significant predictor of small for gestational age. Chronological age has an even greater impact: women younger than 15 years are 5.7 times more likely to have low birth weight infants. However, chronological age did not increase the likelihood of having an SGA newborn.

Gestational weight gain (GWG) and birth weight (BW) have a multifactorial etiology, which makes identifying the most influential determinants difficult. The association between variants of the FTO and LEPR genes has been explored as contributing factors to obesity in various age groups; however, their role in GWG and BW in adolescent mothers and their offspring is uncertain. To determine whether the presence of polymorphisms rs9939609 (FTO) and rs1137101 (LEPR) is associated with gestational weight gain and newborn weight in a cohort of adolescent mothers. Methods: A prospective cohort study of 305 mother-child dyads was conducted between 2020 and 2024. Genotyping of the single nucleotide variants (SNVs) rs9939609 of the FTO gene and rs1137101 of the LEPR gene was performed using real-time PCR and high-resolution melting analysis (qPCR-HRM), using maternal peripheral blood and umbilical cord blood samples. GWG, BW, energy intake, and other perinatal data were recorded and classified. Genetic data from 305 mother–offspring dyads were analyzed. The median maternal age was 16 years, and 71.4% had a normal pre-pregnancy body mass index (BMI). The most frequent genotypes were TT for FTO rs9939609 and AG for LEPR rs1137101. In both groups, the genotypic distribution significantly deviated from Hardy–Weinberg equilibrium (p < 0.0001). The AA genotype of FTO was associated with a higher probability of excessive gestational weight gain (GWG) after adjustment for pre-pregnancy BMI and dietary and sociodemographic factors. High protein and lipid intake increased the risk of excessive GWG, whereas adequate intake of carbohydrates and legumes showed a protective effect. An initial significant association was identified between the LEPR rs1137101 variant (AA allele) and low birth weight (LBW); however, this association was lost after adjustment for confounding factors. The FTO rs9939609 variant was significantly associated with GWG. On the other hand, the LEPR rs1137101 variant in the offspring showed an association with BW categorized by percentiles (in crude analysis), while the FTO variant showed no relationship with birth weight.