BACKGROUND: This study evaluate the influence of weight gain of pregnant women under different clinical conditions (Tobacco, Diabetes mellitus (DM), Hypertension, Intrauterine growth restriction (IUGR) and Control) on the birth weight of newborns. METHODS: This is an observational, longitudinal study nested in a prospective controlled cohort and conducted from 2011 to 2016 in three hospitals in the city of Porto Alegre (Brazil). Sociodemographic, prenatal (maternal gestational weight gain, among others) and perinatal (birth weight) data from 372 mother-child pairs were analyzed. Pearson's correlation was used to verify the relationship between gestational weight gain and birth weight. Multivariate regression was performed to determine the association between maternal weight gain in the presence of various health conditions and birth weight. RESULTS: There were no statistical differences in gestational weight gain in different health condition groups (p=0.092). However, women in the Hypertension group had a tendency to gain more weight (p = 0.097). Mothers in the intrauterine growth restriction group gained less weight. A positive correlation was observed between gestational weight gain and birth weight in the Tobacco (p = 0.003) and Control (p =0.001) groups, which remained positive only in the Control group after adjustment (p<0.004). CONCLUSIONS: Only weight gain in pregnant women without clinical changes during pregnancy seemed to influence the weight of the newborn. It is assumed that other factors, such as metabolic or nutritional, present among smoking, diabetes mellitus, hypertensive or with intrauterine growth restriction pregnant women have different modulations on the intrauterine environment and, therefore, exert a different influence on fetal growth.