Abstract Background Realizing the importance of regular physical activity, particularly in the prevention of chronic diseases and unhealthy weight gain, it is important to study how physical activity changes during and after pregnancy using prospective study designs. The aim of this study was to describe the mode, duration, intensity, and changes in physical activity during pregnancy through one year postpartum among a cohort of women. Methods This study was part of the third Pregnancy, Infection and Nutrition Postpartum Study at the University of North Carolina Hospitals. A cohort of 471 women was followed at 17-22 and 27-30 weeks' gestation and at 3 and 12 months postpartum. The participants reported the mode, frequency, duration, and intensity of all physical activities that increased their breathing and heart rate in the past week. Results Overall physical activity for the cohort decreased from 17-22 weeks to 27-30 weeks of gestation, but rebounded up at 3 months postpartum and remained stable at 12 months postpartum. The mean MET h/wk values for each time point were 24.7 (standard deviation, SD 26.8), 19.1 (SD 18.9), 25.7 (SD 29.3), and 26.7 (SD 31.5). In postpartum, women reported more care-giving and recreational activity and less indoor household activity, as compared to their activity level during pregnancy. Conclusion For health benefits and weight management, health care professionals are encouraged to provide pregnant and postpartum women with information on recommendations of physical activity, particularly regarding the minimum duration and intensity level.