Group B strep (GBS) is a common type of bacteria that lives naturally in the body, most often in the intestines, rectum, vagina, bladder, or throat. Roughly 15% to 40% of pregnant women carry it at any given time, and most never know because it rarely causes symptoms in healthy adults. GBS becomes a medical concern primarily during pregnancy and childbirth, when it can pass to a newborn and cause serious infections.
Where GBS Lives in the Body
GBS, formally called Streptococcus agalactiae, is part of the normal bacterial landscape in many people. It colonizes the lower digestive and genital tracts without causing harm. You can carry it continuously, on and off over months or years, or never at all. Carrying GBS doesn’t mean you have an infection. It simply means the bacteria are present. In healthy adults, the immune system keeps GBS in check, and most carriers experience no symptoms whatsoever.
Why It Matters During Pregnancy
GBS becomes significant during labor and delivery because the bacteria can transfer to the baby as it passes through the birth canal. For the newborn, whose immune system is still immature, GBS can cause life-threatening infections including sepsis (a bloodstream infection), pneumonia, and meningitis.
These infections fall into two categories based on timing. Early-onset disease appears within the first week after birth and most commonly shows up as sepsis or pneumonia. Late-onset disease develops between 7 and 89 days after birth. It tends to progress more slowly and is especially associated with meningitis.
Warning signs in a newborn include fever, difficulty breathing, bluish or pale skin, poor feeding, and unusual limpness or irritability. Meningitis may also cause seizures or a bulging soft spot on the baby’s head. About 4% to 6% of babies who develop GBS disease will die even with treatment. Among those who recover from GBS meningitis, roughly 3 in 10 face lasting effects like hearing loss, seizures, cerebral palsy, or delays in learning to move, speak, or think.
Screening During Pregnancy
Because GBS carriers feel perfectly fine, the only way to know your status is through testing. The American College of Obstetricians and Gynecologists recommends screening during the 36th or 37th week of each pregnancy. This timing is important: GBS colonization can come and go, so a test done too early may not reflect your status at the time of delivery.
The test itself is straightforward. You lie on an exam table while your provider uses a small cotton swab to collect a sample from the vagina and rectum. There are no needles involved and the process takes just a few seconds. The sample is sent to a lab where the bacteria are cultured, and results typically come back within a day or two. In some cases, GBS is also detected through a routine urine test earlier in pregnancy.
How GBS Is Prevented During Labor
If your screening comes back positive, you’ll receive antibiotics through an IV during labor. The goal is to get at least four hours of antibiotics into your system before delivery, which gives the medication enough time to reduce bacteria in the birth canal and reach the baby’s bloodstream through the placenta.
Taking antibiotics earlier in pregnancy doesn’t work because GBS grows back quickly. Oral antibiotics are also less effective than IV delivery for this purpose. Since the widespread adoption of this approach in the 1990s, early-onset GBS disease in the United States has dropped by more than 80%, falling from 1.8 cases per 1,000 live births to 0.26 per 1,000 by 2010. That translates to an estimated 70,000 prevented cases over roughly 16 years.
If you have a penicillin allergy, your care team has alternatives. A mild allergy (no history of a severe reaction) typically means you can safely receive a closely related antibiotic. For more serious allergies, your GBS sample can be tested to find an antibiotic the bacteria will respond to. There is always a workable option.
GBS in Adults Outside of Pregnancy
While pregnancy gets most of the attention, GBS can also cause infections in non-pregnant adults, particularly those over 65 or people with chronic health conditions. Diabetes, heart disease, congestive heart failure, obesity, and a history of cancer all raise the risk. In these groups, GBS can lead to bloodstream infections, urinary tract infections, skin infections, and pneumonia. These cases are less common than neonatal GBS disease but can be serious, especially when multiple health conditions are present.
What Happens if Your Baby Needs Testing
If a newborn shows signs of a possible GBS infection, doctors will typically draw a small blood sample from the baby’s heel. When meningitis is suspected, a lumbar puncture (spinal tap) may be needed. During this procedure, the baby is held in a curled position while a provider uses a thin needle to collect a small amount of fluid from the lower spine. A numbing cream or injection is used beforehand so the baby doesn’t feel pain, and the fluid collection itself takes about five minutes. The spinal fluid is then tested for signs of bacterial infection.
Early detection and treatment with antibiotics greatly improve outcomes for infected newborns, which is why hospitals monitor babies closely when the mother is known to carry GBS or when symptoms appear in the first days of life.