Background and aims Fetal growth is determined partially by maternal characteristics such as pre-pregnancy weight. Maternal pre-pregnancy anthropometry is useful to estimate birth weight and the risk of various adverse perinatal outcomes. We studied the relationship between pre-pregnancy anthropometry and newborn size. We estimated the risk of low birth weight (LBW, <2,500 g) or high birth weight (HBW, ≥4,000 g), preterm (PT), small (SGA) or large (LGA) for gestational age in a birth cohort from Ribeirão Preto, Brazil, in 2010, according to maternal pre-pregnancy body mass index (BMI).

Methods A convenience cohort of 1370 pregnant women living in the city was evaluated between 22–25 weeks of gestation, and their respective newborns. Standardised questionnaires were applied during pregnancy and soon after birth. The dependent variables were LBW/HBW, PT and SGA/LGA. The independent variable was maternal pre-pregnancy BMI, classified as overweight (BMI between 25 and 29.9 kg/m²) and obese (BMI≥30 kg/m²). Logistic regression models were adjusted for biological, sociodemographic and pregnancy-related variables.

Results A high prevalence of overweight-obesity (39.6%) was observed and gestational weight gain was above international recommendations, especially among obese mothers. Overweight and obesity before pregnancy, according to pre-pregnancy BMI showed high risk of HBW (RR 2.1, 95% CI 1.09–3.68 and RR 2.58, 95% CI 1.36–4.91, respectively) and LGA infants (RR 1.96, 95% CI 1:10–3:49 and 3:47 RR, 95% CI 1.95–6.16, respectively), but not with LBW, SGA or PT.

Conclusions In this population, frequencies of overweight and obesity were elevated and were independently associated with HBW and LGA.

Supported by FAPESP and FAEPA.