What Is Strep B? Definition

Group B Streptococcus — often called Strep B or GBS — is a common type of bacteria that naturally lives in the human body. It is not the same as the “strep throat” bacteria, which is Group A. For most healthy adults, Strep B causes no symptoms at all. The concern is specific: if a pregnant woman carries the bacteria, she can pass it to her baby during childbirth, and that can lead to a serious newborn infection. Because of this risk, routine screening and preventive treatment are standard parts of prenatal care in the United States.

What Is Strep B?

Strep B is a bacterium that colonizes the gastrointestinal tract, urinary tract, and genital tract. Up to one in three healthy adults carries it without knowing. It is not a sexually transmitted infection, and it is not a sign of poor hygiene. For the carrier, it is essentially harmless. The medical term for carrying the bacteria without symptoms is colonization.

The danger arises almost exclusively during delivery. A newborn’s immune system is immature, and exposure to the bacteria through the birth canal can lead to pneumonia, sepsis, or meningitis. These are severe conditions that require immediate hospital care. This is why obstetricians screen for GBS late in pregnancy.

How Is Strep B Transmitted to Babies?

The transmission happens during labor and delivery. When the baby passes through the birth canal, it can inhale or swallow fluids containing the bacteria. This is called vertical transmission. Not every baby exposed to GBS becomes infected, but the consequences of infection are severe enough that preventive measures are standard.

There is a second, less common route. In rare cases, GBS can cross the amniotic membranes before labor begins, infecting the baby while still in the womb. This is more likely if the membranes rupture early — meaning the water breaks more than 18 hours before delivery. It is also more likely in cases of preterm labor.

Importantly, GBS is not transmitted through breast milk, casual contact, or shared surfaces. A baby who becomes infected got the bacteria during the birthing process, not after.

Who Is Screened for Strep B?

Routine screening is recommended for all pregnant women in the United States. The screening happens late in pregnancy, typically at 36 to 37 weeks of gestation. The test is simple: a healthcare provider takes a swab of the vagina and the rectum. The swab is then cultured in a lab to see if GBS is present.

The timing matters. The bacteria can come and go. A woman who tests negative at 36 weeks might test positive at delivery, and vice versa. However, the 36-week test is the most reliable predictor of colonization status at delivery. Testing earlier in pregnancy is not useful because the results do not predict the delivery-time status well enough.

Screening is not optional in most hospital systems — it is a standard component of prenatal labs. If you are pregnant, your provider will likely bring it up automatically. If not, ask about it.

How Is Strep B Prevented in Newborns?

The prevention strategy is well established. If a woman tests positive for GBS, she receives intravenous antibiotics during labor. The standard drug is penicillin, or ampicillin as an alternative. Women with a serious penicillin allergy receive a different antibiotic, typically cefazolin or clindamycin, depending on the sensitivity profile of the bacteria.

The timing of the antibiotics is critical. They must be given at least four hours before delivery to be maximally effective. This is why women who test positive are advised to come to the hospital early in labor — not to wait until contractions are strong. If a woman arrives in active labor and has not received antibiotics, the medical team will start them immediately.

There are situations where antibiotics are given without a positive test. If a woman goes into labor before the 36-week screen, if her water breaks early, or if she has a fever during labor, the provider may treat presumptively. These are considered risk factors for GBS transmission.

What Happens if a Baby Gets Strep B?

Early-onset GBS disease occurs within the first week of life, usually within the first 24 to 48 hours. Symptoms include fever, difficulty feeding, lethargy, irritability, and breathing problems. These are nonspecific — they look like many other newborn conditions — which is why any sick newborn gets a full workup.

If GBS is suspected, the baby is treated with intravenous antibiotics in the hospital. With prompt treatment, the vast majority of infected babies recover fully. However, GBS remains a leading cause of neonatal sepsis and meningitis, and it can be fatal if untreated. This is precisely why the screening and prevention protocol exists.

There is also a late-onset form of GBS disease that appears between one week and three months of age. This form is less commonly linked to birth transmission and can sometimes be acquired from the environment. It is rarer than early-onset disease, and the treatment is the same — hospital care and intravenous antibiotics.

Does Strep B Affect Non-Pregnant Adults?

Yes, but much less commonly. In older adults — typically over 65 — and in people with weakened immune systems, GBS can cause urinary tract infections, bloodstream infections, or skin and soft tissue infections. It can also infect bones and joints. These are not related to pregnancy and are treated with antibiotics.

Adults with diabetes, liver disease, or cancer are at higher risk. So are people taking medications that suppress the immune system. In these populations, GBS is treated like any other bacterial infection. There is no screening program for non-pregnant adults, and no vaccine is currently available, though research is ongoing.

Can Strep B Be Treated During Pregnancy?

Oral antibiotics during pregnancy are not recommended as a preventive strategy. Treating the colonization earlier in pregnancy does not reliably prevent the bacteria from returning by the time of delivery. The bacteria can recolonize quickly, so the standard approach is to treat only during labor, when the exposure risk is immediate.

There is one exception. If a pregnant woman has a urinary tract infection caused by GBS, she is treated with oral antibiotics. This is considered a true infection rather than simple colonization, and treatment is necessary regardless of gestational age.

Is Strep B the Same as Strep Throat?

No. This is a common confusion. Strep throat is caused by Group A Streptococcus. The bacteria are related but behave differently. Group A causes throat infections and skin infections. Group B causes newborn infections and infections in older or immunocompromised adults. The screening test and the treatment protocols are entirely different.

Having strep throat does not make you a GBS carrier, and carrying GBS does not protect you from strep throat. They are separate bacteria with separate clinical implications.

What Are the Risks of the GBS Test and Treatment?

The screening test itself is a simple swab — there is no risk beyond mild discomfort. The antibiotics given during labor are considered safe for both mother and baby. Penicillin and ampicillin have been used in pregnancy for decades, and their safety profile is well established.

The main downside of the protocol is that many women who carry GBS receive antibiotics even though their baby would not have become infected. Most babies exposed to GBS do not get sick. This means the prevention strategy treats many to protect the few who would otherwise develop severe disease. That trade-off is accepted because the consequences of newborn GBS infection are so serious.

Another consideration is antibiotic resistance. Some GBS strains are resistant to clindamycin, which is why sensitivity testing is performed when a woman has a penicillin allergy. This is a real and monitored issue, but it does not change the current recommendation for universal screening and intrapartum prophylaxis.