What Is the Group B Strep Test in Pregnancy and Why Does a Positive Result Matter?

The group B strep test is a swab, usually done late in pregnancy, that checks whether the bacterium Streptococcus agalactiae lives in your vagina or rectum. A positive result usually indicates that you carry it without symptoms, which is common and has nothing to do with hygiene. It matters because the bacteria can pass to the baby during birth and sometimes cause serious infection. This article explains the test, what a positive result changes in labor, and which symptoms need urgent care.

What the test actually looks for

Group B Streptococcus (GBS) is a common bacterium. MedlinePlus (NIH) lists group B strep among the streptococcal infections and notes that it can cause illness in newborns. In adults, many people carry it in the intestine or the lower genital tract and never know, because it usually causes no symptoms.

Carrying GBS is called colonization. It is not a sexually transmitted infection, it is not caused by poor hygiene, and it is not something you did wrong. The bacteria can come and go over time, which is one reason the test is done close to delivery rather than early in pregnancy.

How the swab is done

The test is quick. Your clinician uses a soft cotton swab to collect a sample from the vagina and the rectum. It is usually painless, though it can feel a little uncomfortable. You do not need to take anything off beyond what you would for a routine exam, and in some practices you may be offered the option of collecting the sample yourself.

The swab goes to a lab, where it is placed in a culture medium to see whether group B strep grows. Results often take a couple of days. Screening is usually done in the last several weeks of pregnancy, and your clinician will confirm the timing that fits your situation.

What a positive result means, and what it does not

A positive result means the bacteria were found in the sample on that day. It does not mean you are sick, and it does not mean your baby has an infection. Most people who test positive feel completely well, and many babies born to carriers stay healthy.

The reason a positive result matters is the delivery itself. During labor and birth, a baby can be exposed to the bacteria in the birth canal. In some babies this leads to infection, which can include sepsis (a serious body-wide response to infection), pneumonia, or meningitis. MedlinePlus (NIH) describes sepsis as a life-threatening reaction to an infection that needs prompt medical care. Most exposed babies do not become ill, but because a serious infection is possible and can progress quickly, clinicians plan ahead.

A negative result

A negative culture suggests the bacteria were not detected at the time of testing. A recent negative screen may mean GBS-directed antibiotics are not planned for labor, and your clinician will tell you how long a negative result is considered current. Your team may make a different plan if risk factors appear. No test result removes risk completely, so tell your team if anything changes before delivery.

What happens in labor if you test positive

The usual plan is IV antibiotics once labor begins or your water breaks. Antibiotics are also planned if you had a previous baby with GBS infection or GBS in your urine this pregnancy, and your team may offer them if your status is unknown and risk factors such as preterm labor or fever are present. The goal is to lower the amount of bacteria around the baby during birth. Intravenous penicillin (or ampicillin) is the usual first-choice antibiotic for this purpose unless you have a penicillin allergy, and your team will choose the drug.

Why not treat earlier with pills? Carriage can return after a course of oral antibiotics, so oral treatment before labor does not reliably clear the bacteria by delivery day and is not a substitute for IV antibiotics in labor. Antibiotics during labor target the period when the baby is exposed, and they work best when there is enough time for them to be given before delivery, so your team will usually start them when you arrive in labor. If you think labor may be starting, it is reasonable to call early rather than wait at home.

Tell your team about a penicillin allergy

If you have ever had a reaction to penicillin or a related antibiotic, tell your clinician at a prenatal visit, not for the first time in the delivery room, so an alternative can be chosen early. Reactions can range from a mild rash to swelling of the face or throat and trouble breathing. If you have a penicillin allergy, your team will choose another antibiotic based on your reaction history and, when available, lab testing of your GBS sample, and may suggest allergy evaluation if the history is unclear.

Planned cesarean births

If a cesarean is planned, labor has not started and your water has not broken, GBS antibiotics are often not needed for that reason alone, but your clinician will advise what applies in your case. Ask your care team how a positive result changes the plan for your specific situation.

When your water breaks or labor starts early

If you are GBS positive and your water may have broken, you have regular contractions, or your baby is moving much less than usual, call your labor unit right away, day or night, and go in as they direct, especially before 37 weeks. Our article on how to tell if your water has broken covers the signs of ruptured membranes. For a positive GBS result, the extra point is that antibiotics need time to work, so do not wait to see whether contractions start by themselves or for an appointment.

Two situations need 911 instead of a phone call. One is heavy vaginal bleeding. The other is a cord you can see or feel in the vagina after your water breaks, which is called cord prolapse. While you wait for help, get onto your hands and knees with your hips raised higher than your head, and do not try to push the cord back in.

GBS in a urine test

GBS sometimes shows up in a urine culture during pregnancy, even if you have no symptoms. Tell your clinician soon if this happens. Whether and how it is treated depends on your clinician's judgment and the urine culture result, and it is a reason you will usually be offered antibiotics in labor. Our article on UTIs in pregnancy explains why urinary infections need prompt attention. MedlinePlus (NIH) also lists infections that matter in pregnancy, which can help you see where GBS fits.

What happens to the baby after birth

If you tested positive, the newborn team may watch your baby more closely for a period after birth. How long and how closely depends on factors such as whether you received antibiotics in labor, how early the baby arrived, and whether your water broke well before delivery. Some babies need only routine observation, while others may have blood tests or stay in the hospital longer. Ask the team what monitoring is planned for your baby.

Warning signs in a newborn, even after you go home

GBS illness in babies can appear in the first days, and it can also appear later, over the first weeks to months. Signs can include:

• Fever, or an unusually low body temperature.
• Poor feeding or refusing to eat.
• Unusual sleepiness, floppiness, or being hard to wake.
• Fast breathing, grunting, or pauses in breathing.
• Skin that looks blue, gray, or very pale.
• Irritability that cannot be soothed, or a seizure.

A baby under 3 months with a rectal temperature of 100.4°F (38°C) or higher, a low temperature, poor feeding, or unusual sleepiness or irritability needs emergency evaluation right away. Go to the emergency room now, and call 911 if the baby is struggling to breathe, blue or gray, limp or unresponsive, or having a seizure.

When the parent should worry

Infection in the parent after delivery is possible, including in the uterus or the bloodstream. Fever of 100.4°F (38°C) or higher, shaking chills, belly pain, foul-smelling discharge, or feeling very unwell after birth needs same-day evaluation, and emergency care if you look or feel very sick. Confusion, a very fast heartbeat, severe shortness of breath, or fainting can be signs of sepsis and need 911. Heavy bleeding (soaking a pad in an hour), chest pain, especially with shortness of breath or fainting, a severe headache that does not go away, vision changes, or a seizure after delivery also need 911.

Questions to bring to your next visit

• When will my screening swab be done, and how will I get the result?
• If I am positive, what is the plan when labor starts or my water breaks?
• Does my allergy history change which antibiotic is appropriate?
• What monitoring is planned for my baby after birth?

A positive group B strep test is a planning tool. With antibiotics in labor and attentive follow-up, many people with a positive result deliver healthy babies, though outcomes vary and no step removes risk entirely.

When to Seek Medical Care