The placental transfer of ampicillin.

Cord blood ampicillin levels were assayed in 23 neonates whose mothers received the antibiotic by the obstetrical service. The levels ranged from 2.9 to 36.2 micrograms/ml. Nineteen (82.6%) of the 23 had a serum ampicillin level in excess of 5 micrograms/ml at delivery, which is significantly greater than the minimal inhibitory concentration (MIC) to inhibit ampicillin-sensitive enterobacteriacae and far exceeds the MIC for Group B beta-hemolytic streptococci. Antenatal ampicillin therapy results in significant levels in the neonate that may obscure cultures obtained after delivery.