In a population of 1,050 pregnant women the effect of maternal colonization by group B Streptococcus on premature rupture of membranes (PROM), preterm delivery, and low weight was analyzed. A significant increment was found of the prevalence of PROM for patients colonized in the vagina and/or the rectum (26.4%) versus noncarrier patients (17.8%). In vaginal and/or rectal group B Streptococcus carriers, in whom group B Streptococcus was also isolated from the cervical culture, the rate of PROM was higher (41.7%), while when the cervical culture was negative, the PROM was similar to noncarriers. There were no significant differences with respect to colonization conditions regarding the incidence of preterm delivery or the different preterm delivery indicators analyzed.