How It Spreads and Risk Factors

How it Spreads

Bacteria called group B Streptococcus (group B strep, GBS) come and go naturally in people’s bodies. If a pregnant woman has the bacteria in her body, she can pass them to her baby during labor and delivery. Most babies who get GBS disease in the first week of life (early-onset disease) get it this way. It can be hard to figure out how babies got the bacteria if they get sick later (late-onset disease). The bacteria may come from the mother during birth or from another source. CDC collects information on babies with late-onset disease in 10 states to better understand how GBS bacteria spread.

Smiling pregnant women holding her belly with both hands

About 1 in every 4 pregnant women carry group B strep bacteria in their body.

Risk Factors

On average, about 900 babies in the United States get early-onset GBS disease each year. Some pregnant women are at increased risk of having a baby who develops early-onset GBS disease. Risk factors include:

  • Testing positive for GBS bacteria late in the current pregnancy (35-37 weeks pregnant)
  • Detecting GBS bacteria in urine during the current pregnancy
  • Delivering the baby early (before 37 weeks of pregnancy)
  • Developing a fever during labor
  • Having a long time (18 hours or more) pass between when their water breaks and delivery
  • Having a previous baby who developed early-onset GBS disease

Researchers do not understand the risk factors for late-onset GBS disease as well as those for early-onset disease. Late-onset disease is more common among babies who are born prematurely (before 37 weeks of pregnancy). Babies whose mothers tested positive for GBS bacteria also are at increased risk of late-onset disease.