No abstract

To determine the value of prenatal cultures in defining maternal colonization status at delivery, 5,586 pregnant women were screened at prenatal visits for vaginal and rectal carriage of group B streptococci (GBS). GBS were isolated from 1,272 (22.8%). At delivery, semiquantitative cultures were obtained from 393 prenatal carriers, of whom 264 (67.2%) retained carriage at delivery. Seventeen (8.5%) of 200 women with negative prenatal cultures acquired carriage. The predictive value of a positive prenatal culture was highest (72.5%) in women with prenatal vaginal and rectal colonization and lowest (59.7%) in women with only rectal colonization. The predictive value varied inversely with the interval between prenatal sampling and delivery. In mothers with prenatal carriage, density of colonization at parturition was not predicted by the sites of prenatal colonization. Density of colonization, however, strongly influenced rates of vertical transmission to neonates and rates of heavy infant colonization. Ten infants born to prenatally cultured mothers developed group B streptococcal early-onset disease; the mothers of eight (80%) of the 10 had prenatal colonization with the homologous GBS serotype.

Most cases of neonatal group B streptococcal disease with early onset have an intrapartum pathogenesis. Attack rates are increased substantially in infants born to mothers with prenatal group B streptococcal colonization and various perinatal risk factors (premature labor, prolonged membrane rupture, or intrapartum fever). In a randomized controlled trial, we studied the effect of selective intrapartum prophylaxis with ampicillin in 160 such high-risk women. In infants born to mothers who received intravenous ampicillin during labor, as compared with controls who received no treatment, neonatal colonization with group B streptococci was present in 8 of 85 (9 percent) versus 40 of 79 (51 percent; P less than 0.001), colonization at multiple (greater than or equal to 3) sites was observed in 3 of 85 (4 percent) versus 24 of 79 (30 percent; P less than 0.001), and bacteremia occurred in none of 85 versus 5 of 79 (6 percent; P = 0.024). The side effects of ampicillin were limited to a single episode of urticaria in a mother who had no history of penicillin allergy. We conclude that intrapartum ampicillin prophylaxis in women with positive prenatal cultures for group B streptococci who have certain perinatal risk factors can prevent early-onset neonatal group B streptococcal disease.