The relationship of placental size to perinatal outcome was investigated in a population of low-risk infants. A trimmed and drained placenta was weighed for each of 417 low-risk infants, and for 108 infants whose intrapartum course was complicated only by compression of the umbilical cord. Tracings from intrapartum electronic fetal heart rate monitoring were analyzed by an investigator who was unaware of the fetal weight/placental weight ratio. The incidence of perinatal problems was increased in those infants whose fetal weight/placental weight ratio was greater than 11: intrapartum fetal distress, 20% (p = 0.0046); meconium-stained amniotic fluid, 28.9% (p = 0.0017); Apgar score less than 7, 11.1% (p = 0.04); and hyperbilirubinemia, 24.4% (p = 0.0008). On the basis of these data, the conclusion drawn was that there is a population of presumably low-risk infants who are at increased risk because they have outgrown their placentas.