GBS-positive delivery rates Objective Stripping GBS carrier status obstetrical procedure GBS Carriers outcome measure STRIP-G Study 542 women 361 GBS-negative study groups GBS-unknown patients 46 GBS-unknown status membrane study design Antepartum Membrane Stripping referral center CI post-term pregnancy intra-partum characteristics Odds Ratio obstetric procedure GBS carriers GBS-Positive vs Group B Streptococcus