The active season of the Common Watersnake (Nerodia sipedon sipedon) in Missouri lasts about 170 days, from mid-April to early October. Females begin vitellogenesis in April, ovulate in June, and give birth in August to September. Reproduction occupies about 130 days (76%) of the active season. Food was present in 71% of vitellogenic snakes, 64% of pregnant snakes, and 43% of postpartum snakes. Coelomic fat mass decreased during vitellogenesis but increased during pregnancy. At parturition, 40% of females had sufficient fat reserves to reproduce the following year. Dry mass of ovulated ova/developing young did not change through pregnancy; however, wet mass increased from 45% in freshly ovulated ova to 78% at parturition. We conclude that reproductive females are not anorexic and that the increase in water content of the embryos adds considerable mass to the mother during pregnancy.