The most common screening test for group B streptococcus (GBS) is performed between 35 and 37 weeks of gestation. Generally, this test is performed at 36 weeks, as it is valid for 5 weeks and, if more time elapses before delivery, it should be repeated.
The GBS test is performed in all pregnancies, even if a negative result was obtained in a previous pregnancy. However, if the woman has had GBS infection during pregnancy or a previous baby with GBS infection, she will not be tested, as she will still receive antibiotic prophylaxis.
In addition, the test should be performed regardless of whether the woman is scheduled to deliver by cesarean section. This is because if the amniotic sac ruptures, the baby could be at risk for GBS infection.