Group B Strep in Pregnancy: Testing and Treatment

About 10 to 30 percent of pregnant women carry group B strep in the vagina or rectum without any symptoms, according to the CDC’s Group B Strep program. The bacterium is harmless to healthy adults but can, in a small number of cases, cause serious infection in newborns exposed during birth. Universal screening in late pregnancy paired with antibiotics during labor for carriers has cut the rate of early-onset newborn GBS disease by roughly 80 percent since screening programs began in the 1990s. If you have just tested positive, the most important thing to know is reassuring: this is common, expected, and well managed.

This guide explains what GBS is, how testing works, what a positive result actually means, how antibiotics during labor prevent newborn infection, and what happens if labor moves faster than the testing or treatment timeline. For broader pregnancy care, see our prenatal care guide and our medical conditions library. Everything here is general education — your OB or midwife directs your care.

What Group B Strep Is

Group B Streptococcus, also called Streptococcus agalactiae, is a common bacterium that lives in the gastrointestinal and genital tracts of many healthy adults. It is part of normal flora and rarely causes any problem for the pregnant person. Carriage can fluctuate over time — some people are persistently colonized, others only intermittently. GBS is not a sexually transmitted infection, and being a carrier is not something you did wrong or can “catch up” on.

The concern is vertical transmission during birth. A baby passing through a colonized birth canal can develop early-onset GBS disease, which appears in the first week of life as sepsis, pneumonia, or meningitis. Without prevention, a small percentage of babies born to colonized mothers develop early-onset disease, and it can be serious — which is exactly why the screening-and-antibiotics system exists and why it works so well.

How GBS Is Tested

The standard approach in the U.S., recommended by the ACOG Committee Opinion 797, is a universal vaginal-rectal swab between 36 weeks 0 days and 37 weeks 6 days of gestation. The swab is inserted first into the lower vagina and then into the rectum. Your provider can collect it, and many practices now offer self-collection in the office. It is quick and not painful.

Standard cultures take about 24 to 72 hours to result. Newer rapid PCR-based tests can result within hours and are increasingly used for people who arrive in labor without prior screening or with unknown status. A screening result is generally considered valid for about five weeks; if delivery has not occurred by around 41 weeks, repeat testing is sometimes considered.

Some people have GBS in the urine (GBS bacteriuria) at any point in pregnancy. This finding signals heavier colonization and is treated as a positive screen regardless of the late-pregnancy swab result. GBS bacteriuria at higher counts is also treated as a urinary infection during pregnancy. This is the one situation where antibiotics are used before labor — and it is directed by your OB.

What a Positive Result Means

A positive GBS culture means antibiotics will be recommended during labor to protect your baby. It does not mean your pregnancy is high-risk in any other way, and it does not require treatment before labor. Importantly, taking oral antibiotics during pregnancy to try to “clear” GBS does not eliminate carriage and does not reduce newborn risk, so it is not done (outside of GBS bacteriuria). The plan is simply: positive test now, IV antibiotics when labor starts.

For anyone who has had a previous baby with invasive GBS disease, antibiotics during labor are recommended in later pregnancies regardless of the current culture result. The same is true for GBS bacteriuria in the current pregnancy. Your OB will flag these situations for you.

Intrapartum Antibiotics

The preferred antibiotic for GBS prophylaxis is intravenous penicillin G, started once labor begins or membranes rupture and repeated at intervals until delivery. Ampicillin is an accepted alternative. For people with a non-severe penicillin allergy, cefazolin is used. For severe penicillin allergy with a history of anaphylaxis, clindamycin is used if the GBS isolate is confirmed susceptible, and vancomycin is the backup. Which antibiotic you receive is chosen by your OB based on your allergy history and, when needed, susceptibility testing.

Effectiveness is best when at least one dose is given at least about four hours before delivery. Adequate, well-timed antibiotics reduce early-onset newborn GBS disease by roughly 80 to 90 percent. Babies whose mothers received timely antibiotics typically just have routine newborn observation rather than extra testing.

Antibiotics during labor do not appear to harm the newborn in any meaningful long-term way. Some studies suggest small, temporary differences in the baby’s early gut flora, but no consistent lasting effect has been shown, and the protection against newborn sepsis is well established. Research on the microbiome continues.

If Status Is Unknown at Delivery

Several situations can leave GBS status unknown at the time of labor: preterm labor before the standard screening window, little or no prenatal care, lost or pending results, or rapid labor after membranes rupture. In these cases, ACOG and CDC guidance recommend antibiotic prophylaxis if any of these risk factors are present: delivery before 37 weeks, membrane rupture lasting 18 hours or more, fever during labor (100.4°F/38°C or higher), or a prior infant with invasive GBS disease.

If rapid PCR testing is available, it can be used during labor to check status when it is unknown. Otherwise, the risk-factor approach guides the decision. Either way, the goal is the same: make sure a baby who could benefit from antibiotics gets them in time.

Effects on the Newborn

Despite prevention programs, GBS remains a leading cause of newborn sepsis, which is why the system matters. Early-onset disease (within the first week of life) is largely preventable with intrapartum antibiotics. Late-onset disease (from about one week to three months) is not prevented by labor antibiotics; it appears to come from other sources and can present weeks after birth with sepsis or meningitis.

Symptoms of GBS disease in a newborn include difficulty breathing, fever or unusually low temperature, poor feeding, lethargy, and irritability. Any newborn with these signs needs urgent evaluation, regardless of the mother’s GBS status or antibiotic exposure — trust your instincts and seek care.

When to seek emergency care: Take a newborn to the emergency room or call 911 if the baby has difficulty breathing, a fever (rectal temperature 100.4°F/38°C or higher in the first three months), unusual sleepiness, refusal to feed, vomiting, or unusual irritability. These can be signs of serious infection and need immediate evaluation.

Special Situations

People having a planned cesarean before labor begins and before membranes rupture do not need GBS prophylaxis, because the baby is not exposed to vaginal flora. Your OB will still confirm the plan with you.

Penicillin allergy deserves careful evaluation. Many people who report a penicillin allergy do not actually have a severe, IgE-mediated allergy, and allergy testing — when feasible during or before pregnancy — can clarify whether penicillin or cefazolin is safe. From a microbiologic standpoint those are preferred over clindamycin or vancomycin, so it is worth asking your OB whether testing makes sense for you.

Home births and birth-center deliveries follow the same principles when possible. Some midwives can provide IV antibiotics in these settings; transfer to a hospital may be needed if the timing window for adequate prophylaxis cannot be met or if other risk factors emerge. Discuss the plan with your birth team ahead of time.

Frequently Asked Questions

Can I be retested if I test positive?

Retesting later in pregnancy generally does not change the recommendation. Once a screening test is positive in the current pregnancy, antibiotics during labor are recommended. Testing is usually repeated only in unusual situations, such as delivery delayed well past 41 weeks.

Are antibiotics during labor safe for the baby?

Yes, in standard doses for the standard duration. Penicillin and cefazolin have decades of safety data. Some research suggests minor, short-term changes in newborn gut flora, but no consistent long-term harm has been shown, while the protection against newborn sepsis is well established.

What if I am allergic to penicillin?

Cefazolin is used for non-severe allergies. For severe (anaphylactic) allergies, clindamycin is used if the GBS isolate is susceptible; otherwise vancomycin. If you report a penicillin allergy, your provider may recommend allergy testing, since many reported allergies turn out not to be true allergies.

Can I take antibiotics before labor to get rid of GBS?

No. Treating GBS with oral antibiotics during pregnancy does not eliminate carriage and does not lower the newborn’s risk, so it is not done — except for GBS found in the urine, which your OB treats separately. The protective step is IV antibiotics once labor begins.

Does GBS go away after pregnancy?

Colonization status naturally fluctuates. Some people clear it; others remain colonized. Carriage is harmless to non-pregnant adults in most cases. The main relevance of GBS is perinatal screening and treatment, which apply during pregnancy and birth.

The Bottom Line

GBS screening in late pregnancy is a brief, painless swab that lets your OB or midwife identify carriers and protect newborns with antibiotics during labor. It has been one of the most successful infection-prevention programs in modern obstetrics. A positive test means antibiotics in labor — not anything alarming about your pregnancy. The practical steps that matter are simple: get screened around 36 to 37 weeks, arrive at labor with your result available, and make sure the antibiotic timeline is respected. When in doubt, ask your OB or midwife.