This study determined the relationship of maternal weight gain in pregnancy to its outcome. Mothers who were overweight at the start of pregnancy had the fewest fetal and neonatal deaths with a 16 pound weight gain at term. The optimal weight gain for normally proportioned mothers was 20 pounds and for underweight mothers 30 pounds. For all three groups perinatal mortality rates increased with weight gains less or more than these optimal values. Very low or very high pregnancy weight gains had only a modest influence on the frequency of common placental and fetal disorders. However, once one of these disorders was established, mortality rates from it usually increased severalfold when mothers had very low or very high weight gains.