Research was conducted at the Maternal Breastfeeding Incentive Center (Centro de Incentivo ao Aleitamento Materno - CIAMA) in São Sebastião, São Paulo, Brazil. The 66 primigravidas were allocated to two groups: a control group, with 29 pregnant women who had not performed any kind of physical activity during pregnancy, and another group, the exercise group, with 37 pregnant women who performed supervised aerobic physical exercises twice a week. These volunteers needed to attend at least 20 sessions to be included in the research. No pregnant women in the control group or the exercise group had performed regular physical activity in the year prior to the pregnancy. All participants received verbal information on how the research would be conducted, and after agreeing to participate they signed a free and informed consent term. The physical activity was prescribed by a physical therapist who remained available to address any doubts about the physical activity and about the research. The study followed the baselines of the American Congress of Obstetricians and Gynecologists (ACOG), where the environment temperature hasn't exceeded 28 °C. The pregnant women were told to wear comfortable clothes and to drink water before and during the activity, and not to perform physical activity while fasting. They were also instructed to interrupt the activity in case they felt dizziness, breathlessness, pain, muscle weakness, dyspnea prior to effort or in case they presented calf swelling or pain, bleeding or signs of labor, or, still, if they noticed decrease in fetal movements. Two weeks after birth the volunteers were contacted by phone to collect information on the type of birth.