Objective To evaluate the relation between changes the body mass index (BMI) percentile, reflected in the Atalah curve, and perinatal outcomes. Methods A cross-sectional study with 1,279 women was performed. Data regarding gestational weight, sociodemographic characteristics and perinatal outcomes were collected through medical charts, prenatal card and interviews in the postpartum period. Women could be classified according to the Atalah curve in the following categories: low weight, adequate weight, overweight, and obese. The BMI was calculated at the first and at the last prenatal care visits, and these values were compared. Results An increase in the BMI category according to the Atalah classification occurred in 19.9% of pregnant women, and an increase of 3.4, 5.8 and 6.4 points of BMI were found for women respectively classified in the adequate weight, overweight and obese categories at the first prenatal visit. Women with high school education presented a lower chance of increasing their BMI (odds ratio [OR] 0:47 [0.24- 0.95]). Women who evolved with an increase in the the Atalah classification were associated with cesarean section (OR 1.97–2.28), fetal macrosomia (OR 4.13–12.54) and large for gestational age newborn (OR 2.88–9.83). Conclusion Pregnant women who gained enough weight to move up in their BMI classification according to the Atalah curve had a higher chance of cesarean section and macrosomia. Women classified as obese, according to the Atalah curve, at the first prenatal visit had a high chance of cesarean section and delivering a large for gestational age newborn.