In an ongoing prospective study at 2 hospitals, 114 cesarean section patients were studied to determine whether giving ampicillin after umbilical cord clamping is as effective as perioperative ampicillin in reducing maternal postoperative morbidity. The same patients have also been studied to determine the effect of prophylactic cesarean section ampicillin on the newborn. Results have shown that prophylactic ampicillin initiated after cord clamping is as effective in reducing maternal morbidity as ampicillin initiated prior to the surgery. There is also no evidence from our study that the transplacental passage of prophylactic ampicillin increases immediate or delayed neonatal infections.