We investigated the association between high parity and fetal morbidity outcomes. We analyzed 22,463,141 singleton deliveries at 20 weeks or more of gestation in the United States from 1989 through 2000. Adjusted odds ratios generated from logistic regression models were used to approximate relative risk for neonatal morbidity in women with 1-4 (moderate parity or type I; referent group), 5-9 (high parity or type II), 10-14 (very high parity or type III) and 15 or more (extremely high parity or type IV) prior live births. Main outcome measures included low and very low birth weight, preterm and very preterm birth, and small and large for gestational age delivery. The overall crude rates for low birth weight, very low birth weight, preterm birth, very preterm birth, and small and large for gestational age were 55, 11, 97, 19, 83, and 129 per 1,000 live births, respectively. The adjusted odds ratios for low birth weight, very low birth weight, preterm, and very preterm delivery increased consistently and in a dose-effect fashion with ascending parity (P for trend < .001). In the case of large for gestational age delivery, the adjusted odds ratio showed an inverted-U pattern, being highest among women in the type III parity cluster. The findings with respect to small for gestational age were inconclusive. High parity is a risk factor for adverse fetal outcomes. However, the impact of heightened parity is more manifest as shortened gestation rather than physical size restriction. These findings could prove beneficial for counseling women of high parity. II-3. © 2005 Lippincott Williams & Wilkins, Inc.