Pregnancy Bliss | Reproductive Health Answers

Published: Tuesday, February 15, 2011 - 13:32

There is a wide divergence in the philosophy of how to deal with Group B streptococcal ‘infection’ for pregnant women between Europe and North America. While European doctors actively discourage testing for the bacteria in pregnancy (for reasons discussed in greater detail here:), in the United States and Canada, testing for Group B strep is done routinely in the third trimester. Now, a study presented at the Society for Maternal-Fetal Medicine's (SMFM) annual meeting The Pregnancy Meeting™, in San Francisco, advocates going further.

The researchers presented findings that show that many women are having different test results for Group B streptococcus (GBS) between their routine third trimester screening and a rapid test performed at the time of labour.  In other words, there are women with negative results in the third trimester but whose results are positive in labour (and vice versa)

GBS early-onset sepsis is a leading cause of neonatal infection. GBS is a bacterium that normally exists in the genital tracts of one-third of women; this bacterium comes and goes in the genital tracts of many women - often without women knowing that they have it. It can lead to an infection and can put an infant at risk if it is present in the mother during delivery. Women are routinely tested for GBS in their third trimester. If the bacterium is present, these women are given antibiotics during labour. Two days are needed to obtain results from the standard culture, which prevents women from being screened in labour. However, a new rapid test that returns results in approximately one hour can be administered at the time of labour.

"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, M.D, 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 labour."

Young and her colleagues enrolled women in their study who presented to labour and delivery with an antepartum GBS culture. GBS cultures and rapid tests were performed during labour and compared to the third trimester GBS culture results

Among 559 women, GBS prevalence was 19.5% with the third-trimester culture and 23.8% with culture performed on samples collected during labour. Compared with the culture obtained during labour, the third-trimester culture correctly predicted GBS positivity at the time of labour only 69% of the time versus the rapid test which correctly predicted GBS positivity 91% of the time. The incidence of GBS discordance from the late third trimester to labour was 10%. Compared with women who identified as Caucasian, African-American and Hispanic women were significantly more likely to have discordant culture results.

The study found that the rapid GBS test used when women present in labour has excellent test characteristics and may be a useful adjunct to the third trimester culture for accurately detecting the presence of GBS at the time of labour.

"We found that African-American and Hispanic women were significantly more likely to have results that converted from the third trimester to the time of labour, so the use of the rapid test might be even more beneficial in those groups," said Young.

Rapid Group B Strep (GBS) Test advocated in labour (North America)

By Dr Joe Kabyemela, MD