OBJECTIVE: Evaluate the relationship between the pregestational body mass index and the gestational weight gain with the birth weight. MATERIALS AND METHODS: Cross-sectional analytical study. Through probabilistic sampling, 197 pregnant women and their neonates participated, whose delivery was attended at the San Juan de Lurigancho Hospital, 2018.The pregestational body mass index was classified as low weight, normal, overweight and obese; the gestational weight gain in insufficient, adequate and excessive; and the birth weight in grams. The variation of birth weight according to pregestational body mass index and weight gain were evaluated using the Kruskal-Wallis and ANOVA H tests, respectively. Similarly, a multiple linear regression model is required, a 95% confidence level is considered. RESULTS: The patients who started the pregnancy with obesity and low weight presented neonates with greater birth weight (3516.9 ± 480.7 g and 3564 ± 148.5 g, respectively), it was determined that there is not association between the pregestational body mass index and the birth weight (p = 0.753). However, the birth weight was higher in postpartum women with excessive weight gain (3582.9 ± 442.1 g) and lower in those with insufficient weight gain (3278.9 ± 447.9 g), a significant association was demonstrated between weight gain and birth weight (p < 0.001).