Pronounced physical exertion may influence intraabdominal pressure and uterine blood flow, hormonal balance, and nutritional status, all of which are important determinants of embryonic and fetal development and survival. Most of the epidemiologic evidence of reproductive effects from occupational physical activity concerns gestational age/premature birth, birth weight/intrauterine growth retardation, and spontaneous abortion. Strenuous work, especially when involving long hours of standing and walking, seems to increase the risk of preterm delivery. The effect on intrauterine growth and spontaneous abortion risk is less clear. As a single factor, heavy lifting has in most circumstances not been associated with a significantly increased risk of these outcomes. In general, heavy work duties should be avoided, and enough rest periods assured, especially in late pregnancy.