When should pregnant women be cultured for Group B Streptococcus?

Culturing pregnant women for Group B Streptococcus (GBS) is critical for preventing early-onset neonatal infections. The correct timing for this screening is between 36 and 37 weeks gestation. This window is strategically chosen because it allows for the optimal identification of carriers near the time of delivery.

At 36 to 37 weeks, the GBS colonization status of the mother is likely to be stable and reflects the likelihood of transmitting the bacteria to the newborn during labor and delivery. If a woman is found to be positive for GBS during this timeframe, appropriate prophylactic antibiotics can be administered during labor, significantly reducing the risk of the newborn developing a GBS infection.

Screening earlier, as might be considered in other options, may not accurately predict the status at delivery, since GBS colonization can change during pregnancy. Collecting specimens at the time of delivery or only in cases of previous infant infection would not effectively reduce the risk to the newborn in the current pregnancy, as these approaches do not prioritize timely identification and intervention.