Group B Streptococcus (GBS) is a part of the human microbiome and the leading invasive pathogen in newborns. GBS colonizes the intestinal and vaginal tract, and maternal colonization is the most important risk factor for invasive GBS disease in the newborn during the first week of life. A newborn with invasive GBS disease may become severely ill and is at increased risk of neurodevelopmental impairment or death. In adults, GBS may cause disease in pregnant women, the immunocompromised and the elderly. In order to quantify the GBS carriage rate and serotype distribution in a contemporary Norwegian population of women at delivery and to investigate the resulting burden of disease, we carried out a prospective cohort study at Oslo University Hospital Ullevål. In this cohort, 25.9% of women giving birth were colonized with GBS. GBS serotype III was the most frequent colonizing strain, followed by types V and Ia in decreasing order of occurrence. We found that 13.8% of the isolates could not be satisfactorily typed using the latex agglutination test, which is the most common phenotypic method for GBS serotyping. In contrast, when applying a combination of capsular genotyping methods, only 0.7% of isolates were not identified. This suggests that a diagnostic genotyping strategy is preferable to serotyping of the GBS polysaccharide capsule in colonized women at delivery. All strains were susceptible to penicillin, but approximately 10% were resistant to erythromycin and clindamycin, the drugs of choice for women with penicillin allergy in need of antimicrobial prophylaxis at delivery. We further demonstrated that term infants born to colonized mothers were more than 3 times as likely to be transferred to the neonatal intensive care unit compared with infants of noncolonized mothers. The increased risk of transfer was independent of invasive GBS disease, and a third of the infants met criteria for probable early-onset GBS disease. Examining the effects GBS colonization may have on the mother, we found that colonized mothers were almost twice as likely to be diagnosed with peripartum infection compared to non-colonized mothers, independently of gestational age, premature rupture of membranes and obstetrical interventions like cesarean delivery. Our findings provide evidence that maternal colonization with GBS is associated with a considerably greater burden of neonatal and maternal disease than generally recognized, and gives additional support for improvement of preventive strategies, the most important being the development of an effective GBS vaccine.