Sample size and statistical power Investigators will study 200 obese pregnant women, 100 per study group. This sample size was calculated to detect a difference in the prevalence of exclusive breastfeeding (at one month postpartum) in 10 percent (12 to 22%), with a statistical power of 90%, a level of significance >0.05, a nonresponse rate of 50% and 15% of abandonment postpartum. To achieve the sample size, recruitment will be held for 5 months, hoping to incorporate to the study, at least, two women daily. This calculation was performed using information on the number of women who could meet the inclusion criteria of the study.