This review lists the most commonly accepted risk factors for low birth weight (LBW) and offers critical examination of research on the effect of maternal physical activity, both occupational and leisure time, on birth weight. Some studies have shown job related physical “activity” to be related to unfavorable birth outcomes, including premature delivery and LBW. However, most studies have not controlled for socioeconomic status, nor has actual physical activity been well quantified throughout gestation. Similarly, results of the relationship between leisure time physical activity and birth weight are mixed. Current evidence appears to indicate participation in moderate to vigorous activity throughout pregnancy may enhance birth weight, while more severe regimens may result in lighter offspring. Careful quantification of caloric balance during pregnancy in chronic exercisers is needed before further conclusions can be drawn. Indeed, the interaction between energy expenditure (whether job related or leisure time) and caloric consumption must be addressed to determine whether physical activityper se may affect birth outcome, or if it is merely a confounder. Future studies should include randomized trials in which women without a history of chronic physical activity are assigned to either an exercise/physical activity or control group.