Background: Streptococcus agalactiae (group B Streptococcus, GBS) is the most common cause of early neonatal infection, but restricting the diagnosis to culture-positive infants may underestimate the burden of GBS disease. Our objective was to determine whether maternal GBS colonization was associated with an increased risk of transfer of term infants to the neonatal intensive care unit (NICU) and, if so, to estimate the incidence of probable early-onset GBS disease.

Methods: We conducted a prospective cohort study of 1,694 term infants whose mothers had vaginal-rectal swabs collected at delivery. Data collected on each mother and infant included demographics, clinical findings and laboratory investigations. The medical staff were unaware of the maternal GBS colonization status.

Results: A total of 26% of the mothers were colonized. Infants born to colonized mothers did not differ from infants born to non-colonized mothers with respect to birth weight or Apgar score. Altogether, 30 (1.8%) of the term infants were transferred to the NICU. Only 1 infant born to a colonized mother had culture-positive early-onset GBS disease. Infants born to colonized mothers were more than 3 times as likely to be transferred to the NICU compared to infants of non-colonized mothers (3.6 vs. 1.1%; OR 3.4, 95% CI 1.6-6.9, p = 0.001); 5 infants of colonized mothers had probable GBS disease with tachypnoea and raised C-reactive protein (3.0/1,000 live term births).

Conclusions: Maternal GBS colonization is associated with increased risk of transfer to the NICU in term infants. The burden of neonatal GBS disease may be greater than indicated by the number of culture-positive cases.