Group B strep in pregnancy

By trimester

Weeks 1–13

1st trimester

Nothing about group B strep is on the calendar this early. The one exception worth mentioning at a visit is if GBS has ever turned up in your urine, or if a previous baby had GBS illness, because both change the plan later.

Weeks 14–27

2nd trimester

Still no routine screening. Colonization comes and goes, which is the whole reason the swab is held until late pregnancy rather than done early and assumed to hold.

Weeks 28–birth

3rd trimester

This is the trimester it happens. The vaginal and rectal swab is timed so the result stays reliable through delivery, and a positive means IV antibiotics once labor starts.

What does the evidence show for group B strep?

• Between 20 and 30 percent of women in the United States carry group B streptococcus in the genital and gastrointestinal tracts, with global prevalence ranging from 11 to 35 percent. 1
• Universal screening is recommended between 36 and 37 6/7 weeks of gestation, a window chosen so the culture stays valid for about five weeks and covers deliveries through 41 weeks. 1
• The screen uses swabs from the vagina and rectum, and a positive result means antibiotics are given during labor. 2
• Early-onset disease appears within the first seven days of life and late-onset disease from roughly a week to 90 days, and about 1 to 3 percent of colonized newborns develop early-onset disease. 3
• Universal third-trimester screening paired with antibiotics in labor cut early-onset newborn disease from about 1.8 cases per 1,000 live births to roughly 0.23 per 1,000. 1

When should I call my provider about group B strep?

A temperature of 100.4°F (38°C) or higher in labor, uterine tenderness, a fast heart rate in you or the baby, or cloudy and purulent amniotic fluid are the signs of intra-amniotic infection, and the labor team needs to hear about them immediately rather than at the next check. Outside labor, burning with urination or tenderness over the flank matters here, because GBS turning up in urine signals heavy colonization and changes the delivery plan. Call as soon as your water breaks or labor starts: membranes ruptured 18 hours or longer and labor before 37 weeks are both named risk factors for early-onset newborn disease, and antibiotics need lead time. When in doubt, call your obstetric team the same day. Early input almost always beats waiting, and the on-call line exists exactly for this.