This study attempted to determine if placental size has implications for fetal/neonatal health and for subsequent childhood growth and development. 38,351 placentas were trimmed and weighed in a standardized way. The following factors were found to be associated with low placental weight: low maternal pregravid body weight, low pregnancy weight gain, high maternal hemoglobin levels during pregnancy, gestational hypertension, paid employment outside the home during pregnancy, and low parity. Taking these factors into consideration, placentas that were underweight for birth weight were associated with high hemoglobin values in neonates and lower-than-expected body size in later childhood. Overweight placentas, largely a result of villous edema, were associated with the following neonatal evidences of acute antenatal hypoxia: low Apgar scores, the respiratory distress syndrome, neurologic abnormalities, and neonatal death. Some of the neurologic abnormalities persisted so that at patient age of 7 years, they were 33 per cent more frequent when placentas had been overweight than when they had been of normal weight (P less than .001).