Objective: Our purpose was to determine the effect of amniorrhexis on cultures for group B streptococci.

Methods: Intrapartum vaginal cultures for group B streptococci were obtained before and after rupture of membranes in 166 women; in addition, rectal cultures were obtained from the last 35 patients.

Results: Before rupture of membranes 49 of 166 (30%) of the vaginal cultures were positive for group B streptococci; only 28 of 166 (17%) remained positive after rupture of membranes (p = 0.0009). Similarly, before rupture of membranes 10 of 35 (30%) rectal cultures were positive, whereas only three of 35 (0.9%) remained positive for group B streptococci after rupture of membranes (p = 0.03).

Conclusions: The prevalence of positive genital cultures for group B streptococci is significantly lower immediately after rupture of membranes.