Group B Strep

What Group B Strep Is and How It's Found

Group B Streptococcus (GBS), or Streptococcus agalactiae, is a common bacterium that many healthy adults carry in the gut, rectum, and vagina without ever feeling sick. Carrying it is called being "colonized," and it is not a sexually transmitted infection or a sign of poor hygiene. Roughly 1 in 4 pregnant people (often cited in the range of about 10 to 30 percent) carry GBS at any given time, and colonization can come and go, so a test reflects a moment in time, not a permanent status. The reason it matters in pregnancy is not the parent's health but the newborn's: if GBS is present in the birth canal during a vaginal delivery, a baby can be exposed during birth. Most exposed babies are fine, but a small number develop a serious infection, which is why the standard U.S. approach (recommended by groups like ACOG and the CDC) is to screen late in pregnancy and give antibiotics during labor to carriers. Because carriage usually causes no symptoms, GBS is found by screening, not by how you feel: providers typically do a vaginal-rectal swab around 36 to 37 weeks (guidance shifted to this window from the older 35 to 37 weeks), and a positive result simply means GBS was detected, not that you or your baby is sick. GBS can also show up in a urine test at any point in pregnancy, which is treated as a sign of heavier colonization. Results from one pregnancy do not reliably predict the next, so screening is usually repeated each time.

Why It Happens, the Risk to Babies, and Treatment

GBS is part of the normal bacterial mix in many people's bodies, so colonization is usually random rather than something you caused. The concern is that a newborn's immune system is immature, so bacteria harmless to an adult can occasionally cause an invasive infection. Certain situations raise a baby's risk: a positive GBS screen, GBS in the urine this pregnancy, a previous baby who had GBS disease, labor or water breaking before 37 weeks, water broken for a prolonged time (often cited as around 18 hours or more), or a fever during labor. Two timelines help: early-onset disease appears in the first week of life (most often within the first day or two) and is what labor antibiotics are designed to prevent, while late-onset disease appears from about 1 week up to around 3 months of age, is not prevented by labor antibiotics, and can occur even after a negative screen because babies can pick up GBS from other sources.

The mainstay of prevention is intravenous antibiotics given during labor, not before. If you screen positive (or meet certain risk criteria when status is unknown), the team starts IV antibiotics once labor begins or your water breaks, repeated at intervals until birth. The goal is enough antibiotic on board for a stretch of labor, with the often-cited target of at least about 4 hours before delivery for the fullest benefit; even so, getting some antibiotic still helps. Penicillin is the usual first choice, with ampicillin as an alternative, and antibiotics such as cefazolin, clindamycin, or vancomycin are used for a penicillin allergy depending on how severe it is and on susceptibility testing, so tell your team about allergies well before your due date. Importantly, oral antibiotics earlier in pregnancy do not work for this because colonization returns; a planned cesarean done before labor and before your water breaks generally does not require GBS antibiotics for this purpose; and a healthy-appearing baby of a treated parent is usually observed rather than automatically treated.

When to Call the Doctor

During pregnancy and labor, contact your provider or labor unit if your water breaks (whether or not labor has started), if you develop a fever, or if you go into labor before 37 weeks, since these affect whether and when you need antibiotics; if you are GBS positive, make sure the labor team knows, along with any penicillin or other antibiotic allergy. After birth, GBS infection in a newborn is a medical emergency, so call your pediatrician or seek emergency care right away for warning signs in the first weeks to months: fever (in a baby under about 3 months, a rectal temperature of 100.4 F / 38 C or higher is an emergency on its own) or an unusually low temperature; fast, labored, grunting, or paused breathing, or a bluish color; unusual sleepiness or being very hard to wake; poor feeding or refusing to feed; persistent irritability or a high-pitched, inconsolable cry; limpness; vomiting; or a bulging soft spot or stiff body, which can signal meningitis. Trust your instincts: if your newborn seems "off" or is getting worse, do not wait, because GBS infections can move quickly and early antibiotics save lives.

The Bottom Line

Group B strep is a common bacterium that many people carry harmlessly; the issue in pregnancy is protecting the newborn during birth. Routine screening around 36 to 37 weeks plus IV antibiotics in labor for carriers has made serious early-onset GBS disease much less common than it once was. The antibiotics during labor are the proven step, not pills taken weeks earlier, and a positive result is manageable, not a reason for alarm. Because late-onset infection can still happen and isn't prevented by labor antibiotics, knowing the newborn red flags matters regardless of your screening result. This is general information, not a substitute for individual care; ask your obstetric provider about your screening plan and your pediatrician about anything that worries you in your baby's first months.