Implementation of national guidelines for the prevention of group B streptococcal (GBS) infections has led to an increase in intrapartum antibiotic use and reduction in early-onset GBS infections in newborns. Other outcomes, including the clinical diagnosis of sepsis in term infants, treatment with antibiotics, length of stay, and cost have not been described. To examine these outcomes, we performed an analysis of maternal and newborn data collected between 1998 and 2002 of 130 447 in-hospital births of newborns >or=37 weeks gestation and their mothers from a large vertically integrated healthcare organisation in Utah. ⋯ After controlling for maternal chorioamnionitis, delivery by caesarean section and maternal GBS status, newborns of mothers who received antibiotics were significantly more likely to be treated for clinical sepsis than were newborns of mothers who had not received them [adjusted OR = 3.3; 95% CI 2.9, 3.8]. The average length of stay for newborns whose mothers were treated with antibiotics was 55.8 h compared with 41.6 h for those not treated (P < 0.0001). The cost of caring for newborns whose mothers received antibiotics was $740 compared with $638 for those whose mothers had not received them (P < 0.001).

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