Selective intrapartum chemoprophylaxis of neonatal group B streptococcal early-onset disease. III. Interruption of mother-to-infant transmission.

The effect of intrapartum ampicillin treatment on vertical transmission of group B streptococci (GBS) was examined in 575 prenatally colonized parturient women and their 580 newborn infants. Eighty women (43 receiving ampicillin) with premature labor and/or prolonged rupture of amniotic membranes were randomized. The other 495 were stratified into groups of 358 (31 receiving ampicillin) with no perinatal risk factors; 119 (28 receiving ampicillin) with premature labor and/or prolonged membrane rupture; and 23 (18 receiving ampicillin) with intrapartum fever. Ampicillin virtually eliminated vertical transmission in the treatment group with no risk factors and in both treatment groups with premature labor and/or prolonged membrane rupture. GBS colonization of neonates was detected only in women with intrapartum fever or brief (less than 1 hr) duration of treatment prior to delivery. Ampicillin treatment was associated with a highly significant reduction in maternal postpartum vaginal colonization by GBS. There were six group B streptococcal early-onset infections in infants of untreated subjects and no cases in treated subjects.