In 2002, the American College of Obstetricians and Gynecologists revised their 1994 guidelines to recommend that pregnant women without medical or obstetric complications follow exercise guidelines for nonpregnant women. In spite of these guidelines, the optimal dose of recreational physical activity (i.e., total volume of energy expenditure, and specific frequency, duration and intensity) for a pregnant woman remains to be determined. Identification of this dose has been hampered by the complexity of assessing recreational physical activity in general, and particularly during pregnancy, a demanding period characterized by changing physiology. Therefore, we reviewed the measurement methods in the epidemiologic studies that examined the association between recreational physical activity and two specific pregnancy outcomes of primary public health importance, birthweight and length of gestation. From a review of the medical literature, we identified 20 published journal articles between 1966 and 2005. Study designs, participants, and physical activity assessment measures varied widely across studies. The majority of these studies did not consistently assess type, frequency, intensity, and duration of activity and did not measure these variables during each trimester of pregnancy. Because the effect of recreational physical activity on birth outcomes is likely to be modest, measurement must be highly accurate to minimize the possibility that an effect will not be observed because of measurement error. Recommendations are made for the future assessment of recreational physical activity in epidemiologic studies of these major birth outcomes.