OBJECTIVE: We sought to evaluate the effect of antepartum and intrapartum antibiotic use on antimicrobial-resistant neonatal sepsis. STUDY DESIGN: We analyzed perinatal outcomes for 8474 pregnancies (8593 live births) delivered at 6 hospitals. Data were collected regarding maternal antibiotic use and perinatal course, neonatal cultures, and outcomes. The diagnosis of confirmed neonatal sepsis required at least one positive blood or cerebrospinal fluid culture. Neonatal cultures were evaluated on the basis of the occurrence and timing of maternal antibiotic exposure. RESULTS: There were 96 neonates with confirmed sepsis (11.2/1000 live births). Sepsis was 19.3-fold more common after preterm birth (57 vs 3. 1/1000; P <.001), with 76% of septic infants being delivered preterm. Forty-five percent of pathogens were ampicillin resistant. Ampicillin resistance increased with preterm birth (50% vs 26%; P =. 04), antepartum antibiotics (57% vs 34%; P =.03), intrapartum antibiotics (55% vs 28%; P <.01), and any prenatal antibiotic exposure (52% vs 22%; P =.01). Infection with an organism resistant to at least one maternal antibiotic was more common with intrapartum antibiotic exposure than with antepartum exposure only (57% vs 17%; P =.01). Regarding early-onset sepsis (n = 55), ampicillin resistance was more common with intrapartum antibiotics (50% vs 16%; P <.01), and resistance to at least one maternally administered antibiotic was more frequent with intrapartum exposure (56.7% vs 0%; P