Objective: To evaluate GBS colony numbers in the urine at 35-37 weeks' gestation to predict the load of GBS-colonization of the vagina at birth.

Methods: In this prospective observational study, we included 902 unselected pregnant women. Exposure was GBS colony forming units (CFU) per mL urine at 35-37 weeks' gestation. Outcome was vaginal GBS colonization at birth as assessed by a semi-quantitative culture of a vaginal swab sample (negative, +1, +2, +3).

Results: Bacteriuria with GBS at 35-37 weeks' gestation performed with a sensitivity of 30% concerning any degree of vaginal GBS colonization at birth (31 of 104 cases); 19% for light (+1), 17% for medium (+2), and 52% for high load (+3) vaginal GBS colonization. The colony count in case of GBS bacteriuria at 35-37 weeks' gestation performed with positive predictive values of 35% for <104 CFU/mL, 70% for 104 CFU/mL, and 67% for >104 CFU/mL.

Conclusions: Even though the urinary GBS CFU at 35-37 weeks' gestation is strongly associated with a high load of vaginal GBS colonization intrapartum, it may not perform satisfactorily as a standalone-screening marker for risk of early-onset GBS disease.