Little is known about the fetal-neonatal pharmacokinetics of maternally administered, weight-based vancomycin dosing for group B streptococcus (GBS) intrapartum antibiotic prophylaxis. Our objective was to quantify vancomycin concentrations in umbilical cord blood at birth after weight-based maternal intrapartum vancomycin administration and to assess cord blood vancomycin levels relative to the established GBS clinical minimum inhibitory concentration (MIC) breakpoint. Using a convenience sample of stored sera from our biorepository, we measured vancomycin levels in umbilical cord blood from 26 neonates after maternal intrapartum vancomycin exposure. Most neonates (24/26, 92.3%
95% CI, 74.9-99.1%) had vancomycin cord blood levels above the MIC breakpoint (1 microgram/mL or higher) for GBS.