The study included 20 nulliparous in gestational week 16-20 , attending prenatal care at three tertiary hospitals in Colombia who were randomly assigned into one of two groups : The exercise group ( n = 10 ) took part in an exercise session three times a week for 12 weeks which consisted of : aerobic exercise at an intensity of 55-75 % of their maximum heart rate for 60 min and 25 mins .