Studies have found positive correlations between prenatal exposure to testosterone and masculinization of offspring traits, particularly among females. The present study sought to determine if physical or sexual activity by the mother during pregnancy was related to masculinized/defeminized offspring traits in adulthood. Data were obtained from a large sample of North American college students (offspring) and their mothers. Information about maternal activity levels during pregnancy were reported retrospectively by each mother. The offspring provided self-ratings of various sexually dimorphic traits. Several significant correlations were found. By and large, as maternal physical activity increased, feminine mannerisms decreased and masculine mannerisms increased in the offspring, particularly for females. Maternal physical activity was also associated with increased upper- and lower-body strength and especially with adult height among offspring. Sexual activity by the mother was only associated with upper-body strength and adult height, particularly of the female offspring. Several sexually dimorphic physical traits in offspring are associated with maternal activity levels during pregnancy. Prenatal testosterone is almost certainly involved. The associations could either reflect genetic influences (given that prenatal testosterone is highly heritable) or an effect of maternal testosterone being transferred to the developing fetus. More research is needed to assess the relative merit of these two possibilities.