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 In Ask an Expert, Midwifery, OB/GYN & Women's Health, Pulse Blog

Q: Is Group B strep testing necessary?

Group B strep (also called Group Beta strep or GBS) is a bacteria found in some people’s gut microbiomes. The gut microbiome is made up of helpful, harmful, and neutral microbes that influence the immune system.

In most adults, GBS is a non-harmful bacteria. About 30% of people have it in their large intestine. GBS bacteria can come and go throughout an individual’s life without anyone ever noticing. When GBS bacteria are present in the gastrointestinal tract, they are often present in the vagina as well—a result of anatomical proximity.

When babies are born vaginally, they are exposed to the microbiome of the vagina. If GBS is present in the vagina, babies are exposed to it. Unfortunately, babies are born with immature immune systems that make them more susceptible to GBS infection.

Exposure to GBS during birth can cause a newborn to develop an infection called GBS disease. This disease can be very dangerous in newborns and result in neonatal intensive care unit (NICU) admission or neonatal death. GBS-positive people who are not treated with antibiotics in labor have a 50 percent chance of passing the bacteria to their baby. Of babies who are infected with GBS, 1–2 percent will develop GBS disease. Of these babies who contract GBS disease, 4–6 percent will die.

However, the good news is that if a GBS-positive person is adequately treated with antibiotics during labor, the risk of them passing GBS to their baby and their baby subsequently developing GBS disease decreases by more than 80 percent.

To reduce the risk of GBS exposure during childbirth, we test all pregnant people at 36–37 weeks gestation. The test is performed using a swab that is inserted about 2 cm. into the vagina, and then inserted 1 cm. into the anus. The results take about a week.

When individuals test positive for GBS, we recommend treatment with antibiotics during labor. These antibiotics are administered through an IV every four hours until the baby is born. GBS is not treated prior to labor because, as soon as the antibiotics are discontinued, the bacteria re-grows. We suppress the bacteria only when there is a chance that the baby could come into contact with it. For GBS-positive people, receiving antibiotic treatment during labor is an important way that they can help keep their newborn safe and healthy.

—Tilly Field, CNM

Tilly Field believes that every pregnant person deserves to bring their baby into this world on their terms. As a nurse midwife, Tilly focuses on facilitating a safe, nurturing birth experience for families and their newborns.

Matilda (Tilly) Field, CNM.