Objective: To derive and test the implications of a sex-specific fetal growth standard. Design: Secondary analysis of a prospective observational cohort. Setting: Eight U.S. centers. Population or Sample: Nulliparas followed longitudinally through pregnancy. A lower-risk subgroup (exclusions: chronic hypertension, pre-gestational diabetes, suspected aneuploidy, preterm delivery) was selected for fetal growth equation derivation. Methods: Fetal weights at 14-20 weeks, 22-29 weeks, and birth were used to derive a sex-specific fetal growth equation. We compared rates of SGA and LGA by sex using the sex-specific and sex-neutral (Hadlock) standards. Using the full unselected cohort, we assessed outcomes and clinical management according to SGA and LGA status. Main outcome measures: Proportion considered SGA and LGA; obstetric interventions relevant to SGA and LGA. Results: We derived a sex-specific equation using 7,280 infants. The sex-neutral standard diagnosed SGA more often in female and LGA more often in male newborns. The sex-specific standard resolved these disparities. Using the full unselected cohort (N=8,339), newborns reclassified from SGA to AGA by the sex-specific standard were more likely to be delivered for growth restriction with comparable risk of morbidity compared to newborns considered AGA by both methods. Newborns reclassified from AGA to LGA by the sex-specific standard had higher rates of cesarean for arrest of descent, cesarean for arrest of dilation, and shoulder dystocia than newborns considered AGA by both methods. Conclusions: The sex-neutral standard generates sex disparities in SGA and LGA at birth. A sex-specific standard resolves these disparities and may improve growth pathology risk stratification.