Introduction: Pregnancy is a risk factor for the development of obesity. Review studies highlight moderate tests, which support the association between excessive weight gaining during pregnancy and weight retention after delivery. Objective: To assess the relation between the pre-gestational body mass index, the weight gaining and the retention of weight after delivery. Methods: An observational, cross-sectional analytic study was implemented. 91 women, whose deliveries were assisted in December 2016 at the Hospital Marino Molina Scippa-EsSalud, were evaluated. The study variables were pre-gestational body mass index (BMI) and weight retention at the 6th month after delivery. Statistical analysis included the independence of variables by Chi2 and T test. Results: The mean of the pre-gestational BMI and post-delivery BMI were 25, 23 ± 4, 1 y 27, 56 ± 4, 54 respectively. Gain of gestational weight was 12,67; 11,95; 10,42 y 10,00 kg among women with low, normal, overweight and obesity, respectively. Retention of weight after delivery for each group was 4,70 kg (low weight); 6,43 kg (normal); 4, 66 kg (overweight) and 5,92 kg (obesity). Women in the overweight/obesity increased in 50,6 % before pregnancy and 71,5 % six months after delivery. Conclusions: There was a significant difference between pre-gestational IBM and the post-delivery IBM, in all nutritional groups and the groups of weight gaining during pregnancy.