A growing body of literature suggests that healthy and well-nourished women can safely participate in most forms of physical activity during pregnancy. In addition, participation in physical activity during pregnancy may have potential health benefits, such as reducing the risk of gestational diabetes mellitus. These potential benefits are reflected in a recent opinion issued by the American College of Obstetricians and Gynecologists (ACOG), in which women without medical or obstetric complications are recommended to accumulate 30 minutes or more of moderate intensity physical activity on most, if not all, days of the week. Despite these recommendations, little is known concerning current patterns of physical activity among pregnant women. The first study of this dissertation addressed this limitation by providing estimates of the frequency, intensity, and duration of commonly performed activities during pregnancy among a racially and economically diverse sample of women receiving prenatal care. Comprehensive 24-hour recalls were used to measure physical activity in all activity domains and the factors associated with activity were identified. The second study in this dissertation evaluated the reliability and validity of the Kaiser Physical Activity Survey (KPAS) when used to assess pregnancy physical activity. The KPAS assesses all domains of physical activity and is more practical than 24-hour recalls to measure activity in large epidemiologic studies. These results suggest that the KPAS is a reliable and reasonably accurate instrument for estimating physical activity among pregnant women. These findings should help to address the need for valid instruments to assess pregnancy physical activity in etiologic research, as most prior studies have focused on leisure time activity using questionnaires not validated for use among pregnant women. The final study in this dissertation used the KPAS to examine the association between physical activity during pregnancy and gestational weight gain among Hispanic women. This study was unique in its use of a measure of total activity energy expenditure, rather than a measure of leisure exercise, as the exposure of interest. Consistent with prior literature, moderate levels of physical activity was not efficacious in preventing excess weight gain during pregnancy.