Does third-trimester screening
accurately predict GBS in labor?

The prevalence of culture-identified group B Streptococcus in pregnant women rose from 19.5% in the late third trimester to 23.8% at the time of labor, a new study has found.

Senior Editor

A test for group B Streptococcus (GBS) that is negative at the time of late third-trimester screening may not necessarily remain negative until delivery. In a prospective, observational study of 559 pregnant women who underwent routine GBS screening during the late third trimester and again at the time of labor, the rate of conversion from negative to positive results was 10.4%. The findings were presented at the Society for Maternal-Fetal Medicine (SMFM) annual meeting in San Francisco in February 2011.

“Most women are currently screened for GBS in their last trimester, but even if the test is negative, GBS can still develop before women give birth,” said Brett Young, MD, an obstetrician undergoing training in high-risk obstetrics at Massachusetts General Hospital. “We know that two thirds of infants with GBS sepsis are born to mothers with negative third-trimester cultures, so we wanted to see how many women with a negative GBS test in the third trimester have a positive GBS result right before delivery. These babies in particular are at risk for acquiring GBS infection because the mothers are not given appropriate antibiotics in labor.”