Overview

Group B strep (GBS), short for group B streptococcus, is a type of bacterium that naturally resides in the gastrointestinal tract, vagina, and rectum of many pregnant women. GBS is typically harmless and causes no symptoms. During pregnancy, it can pose serious risks if passed to a newborn during birth, potentially leading to life-threatening infections. Due to these risks, routine GBS testing is recommended for all pregnant women. If you test positive, there are established guidelines to protect your baby. Learn what to expect during GBS testing, how a positive result will impact your labor and birth, and explore the latest research on a simple supplement that could potentially reduce the chances of having GBS.

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Group B Strep

Group B strep or GBS is short for group B streptococcus, a type of bacterial infection. This bacterium naturally lives in the gastrointestinal tract, vagina and rectum of many healthy adults. Most women with group B strep do not experience any symptoms, and it is typically harmless. During pregnancy, it presents concerns. In rare cases, GBS during pregnancy can lead to maternal urinary tract infection, intraamniotic infection, or endometritis and is associated with preterm labor and stillbirth. It can cause serious complications if it is passed to a newborn during birth and they develop a GBS infection. Group B strep is the leading cause of newborn infection. Advancements in detecting and treating GBS disease in the United States have significantly improved outcomes. However, despite these efforts, 4% to 6% of babies who develop GBS disease do not survive.

Group B Strep and Your Baby

There are two types of newborn GBS infections, early or late-onset group B strep. Early-onset group B strep is the most common, and symptoms usually show up within a few hours and up to seven days after birth. Symptoms of GBS disease include fever, difficulty feeding, irritability or lethargy (limpness or baby is hard to wake up), difficulty breathing, and blueish color to skin. The most common complications of early-onset GBS are sepsis, pneumonia, and meningitis. This bacteria can also cause breathing problems, heart and blood pressure instability, and gastrointestinal and kidney problems.

Late-onset usually appears within a week up to three months after birth, and meningitis is the most common complication. Late-onset can result from the transfer of bacteria from mom to baby at birth or through contact with someone with GBS.

Testing for Group B Strep

Due to the risks associated with GBS, testing during pregnancy is routine in many countries. Some countries only test pregnant women with clinical risk factors. In the United States, all pregnant women are routinely tested for GBS regardless of other risk factors. The colonization of group B strep can come and go, and you could be positive early in your pregnancy and later test negative. Colonization at the time of labor onset is the most important risk factor for neonatal GBS. To find out if you are colonized near birth and could pass the bacteria to your baby, the test for GBS takes place in week 36 or 37.

This is considered a non-invasive test that requires a swab of your vagina and rectum. The sample is sent to a laboratory, and results are typically available within 24-48 hours. According to the American College of Obstetricians and Gynecologists, the prevalence in pregnant women is between 10% and 30% and is reported to be higher in black women and may vary by geographic location.

A negative group B strep test requires no action or changes to your prenatal care. A positive test indicates that you are colonized with GBS, and your care provider will recommend interventions to reduce the risk of your baby developing a GBS infection.

Opting Out of a GBS Test

As with any procedure during pregnancy, a GBS test should come with informed consent, and you can opt in or out. Since the GBS test is universally recommended in the United States, you will likely get pushback from your care provider if you decline the test.

Treating Group B Strep

Recall that approximately 50% of women colonized with GBS will transmit the bacteria to their newborns. Not all babies colonized with GBS will develop GBS disease. However, it can be a very serious condition for those that do. According to ACOG, without intravenous antibiotics during labor, 1–2% of those newborns will develop GBS early-onset disease. As a result, it is common practice to treat you with antibiotics if you are colonized with group B strep, even if you are considered low-risk. According to the CDC, if you have GBS without antibiotics, there is a 1 in 200 chance your baby will develop GBS disease. With antibiotics, your baby’s risk decreases to a 1 in 4,000 chance.

If your GBS status is unknown, ACOG recommends antibiotics for mothers who have risk factors for GBS. This includes birth before 37 weeks, rupture of membranes for 18 or more hours, a fever of 100.4°F (38°C), or a GBS colonization in a previous pregnancy. If you plan for a home or birth center birth, decline a GBS test, and transfer to a hospital, the hospital may recommend antibiotics as a precautionary measure.

Antibiotic Administration

Group B strep lives in your gastrointestinal tract, and if you treat it with antibiotics before labor, it could potentially return after antibiotics and before you give birth. As a result, if you are positive for GBS, your care provider will administer antibiotics at the beginning of your labor through an IV, then every four hours during active labor until your baby is born.

Penicillin is the antibiotic of choice to treat GBS. If you have a penicillin allergy, your doctor may order a penicillin allergy test to confirm the allergy. Approximately 80% to 90% of people who report a history of penicillin allergy are not truly allergic because the sensitization is lost over time or the original reaction was unrelated to penicillin. If you have a high-risk penicillin allergy, your doctor may order a lab test to determine if the GBS is resistant to alternative antibiotics. The results of these tests allow your care provider to determine the best antibiotic to use.

Continuous IV

The initial dose of antibiotics takes about 15–30 minutes to administer once labor begins. Subsequent doses are typically given every four hours. Receiving antibiotics for group B strep does not require you to be continuously attached to an IV. Between doses, you can have a hep-lock. This capped-off IV needle in the top of your hand allows you to move around freely without being tethered to an IV pole. You could need IV fluids for other reasons unrelated to group B strep, such as hydration. Even if you are continuously connected to an IV for fluids or medications, mobility is still possible using an IV pole on wheels.

Cesarean Birth

If you have a planned cesarean, you do not require antibiotics to treat GBS unless your water breaks. That is because group B strep is not generally passed to a baby in a cesarean birth. However, your doctor will still administer other antibiotics routinely used in a cesarean section to prevent postoperative infection. A review of 95 studies involving over 15,000 women found that routine use of antibiotics at cesarean section reduced the risk of infections in mothers and the risk of severe complications of infections by 60% to 70%. 

Routine testing for GBS is still recommended for women who plan a cesarean birth in the event labor starts or your water breaks before your scheduled cesarean. In those cases, your care provider would recommend antibiotics to treat GBS during the surgery.

Group B Strep and Other Labor Interventions

A positive group B strep test may prompt some care providers to modify recommendations for some other labor and birth interventions. There is a hypothetical concern about bacterial seeding during some methods to induce labor, such as membrane sweeping or utilizing a mechanical dilator. ACOG cites limited evidence and doesn’t have a strong opinion on this. For interventions like vaginal exams and rupturing the membranes, ACOG notes the evidence is mixed and supports those interventions in GBS-positive mothers when they are clinically indicated and with the use of antibiotics in labor. There have been concerns raised about a GBS-positive mother having a water birth. Research shows birthing in water is a safe option even if a mother tests positive for GBS. ACOG supports immersion in water for the first stage regardless of GBS status.

How Antibiotics Affect You and Your Baby

Antibiotics are crucial in preventing and treating infections. The downside of antibiotics is that they don’t only kill the bacteria causing the infection you are trying to treat; they also kill many beneficial bacteria in the body. While antibiotics are considered safe during pregnancy, there are some downsides to their use.

At birth, your baby’s gastrointestinal tract is sterile and quickly becomes colonized by bacteria from both you and their environment. If you take antibiotics during pregnancy, evidence shows they cross the placenta and reach your baby. Numerous studies, like this one, show antibiotic use during labor alters the gut microbiome of newborns. Research shows the use of antibiotics in labor increases the rates of thrush, which is a common yeast infection in mothers and babies during breastfeeding. There is evidence that GBS positive women treated with antibiotics had a modified vaginal microbiota composition with a low abundance of Lactobacillus. Lactobacillus is a dominant bacteria in the vaginas of healthy pregnant women. One study showed that administering antibiotics in labor was associated significantly with a decreased transmission rate of Lactobacillus-dominant mixed flora to neonates.

If you have any concerns about how antibiotics could affect you or your baby, please discuss them with your doctor. In the case of GBS, the benefits of antibiotics to prevent a potential GBS infection likely outweigh the risks of temporary gut microbiome disruption. One of the most beneficial things you can do for your baby’s gut microbiome is to breastfeed. Breast milk provides nutrients and probiotics that help establish a healthy gut microbiome. For more in-depth information and research on antibiotics during pregnancy, birth, and breastfeeding, check out this episode.

Preventing Group B Strep

Ideally, preventing colonization with group B strep altogether would eliminate the risk of passing it to your baby during birth and remove the need for antibiotics during labor. Some interesting research examines the use of probiotics to decrease the incidence of group B strep colonization. The first study to evaluate this utilized panty liners to introduce Lactobacilli bacteria to participants and found high numbers of Lactobacilli may contribute to a low vaginal pH and seem to have a negative influence on group B strep. Since this first study in 2006, more research has been conducted examining taking a daily oral probiotic to prevent GBS.

The Evidence on Preventing Group B Strep

Research continued with a small study in 2014 that found that a prenatal probiotic can potentially reduce group B strep colonization. This study justified a full controlled clinical trial that was completed in 2018. The clinical trial included 251 randomized participants taking probiotics or a placebo. On average, participants took a probiotic for 12 weeks leading up to birth. 18.5% of the group that took the probiotic still tested positive for GBS, compared to 19.7% of the placebo group. Unfortunately, this result was not statistically significant, and the researchers note that larger studies are needed.

A study published in 2020 evaluated the potential of oral probiotics to eradicate a current vaginal GBS colonization. The study participants were women who tested positive for GBS. 33 women in the control group took a probiotic supplement with four strains of Lactobacillus twice daily for 14 days. This was compared to a placebo group of 27 women. In a follow-up GBS test, 63.6% of the control group was GBS positive compared to 77.8% in the placebo group. Although researchers observed a trend toward reduced GBS persistence after probiotic intake, it was not considered statistically significant.

A 2020 review concluded that there is limited evidence to recommend the regular use of probiotics to minimize the risk of GBS colonization. A 2022 systematic review and meta-analysis found probiotic administration during pregnancy, namely in the third trimester, was associated with a reduced GBS recto-vaginal colonization at 35–37 weeks and a safe perinatal profile. The same year, another systematic review and meta-analysis revealed that the use of an antenatal probiotic decreased the probability of a positive GBS result by 44%.

In a more recent 2023 study pregnant women were randomized to take a probiotic or placebo once daily from 28 weeks. Swabs to test for group B strep and microbiome analysis were collected at 28 and 36 weeks. A total of 83 participants completed the study. 20.4% of the placebo group tested positive for GBS at 36 weeks, compared to 15.4% in the probiotic group. Although there was a 5% reduction in GBS colonization, it did not reach statistical significance. Researchers did observe that the probiotic significantly reduced gastrointestinal symptoms of pregnancy compared with placebo. This aligns with much evidence on the benefits of probiotics and probiotic-rich foods.

As of February 2025, a double-blind randomized placebo-controlled clinical trial is underway to investigate whether using three specific species of probiotics taken orally in pregnancy from 25 weeks will reduce the incidence of GBS. Hopefully, that will give us more data on whether probiotics can reduce GBS.

Summarizing the Research

The purpose of reviewing this research’s timeline and diving into each study’s details is to show you how the process unfolds and highlight how long it takes to build strong evidence. Even after research is conducted, it often takes much longer to influence attitudes and recommendations within the medical community. All you can do is make the best decision for you based on the information we have now. Research requires clear standards to determine whether findings are statistically significant. While some studies may not meet this threshold, it’s important to recognize that they still suggest a slight risk reduction. If taking a daily probiotic has the potential to lower your risk of GBS and eliminate the risk to your baby and the need for antibiotics, it is worth considering.

Should You Take a Probiotic?

Although there is not overwhelming evidence to show that taking a probiotic will prevent group B strep, there is substantial evidence of other benefits, especially during pregnancy. Research shows that probiotics can positively impact bowel movements for people experiencing constipation, a common pregnancy symptom. Evidence shows that milk-based probiotic foods containing lactobacilli may decrease the risk for preeclampsia. A recent review found that supplementation with Lactobacilli alone or Lactobacilli with Bifidobacterium seems protective. Probiotics may improve insulin resistance and reduce the risk of gestational diabetes. They found probiotics may influence brain activity to reduce anxiety and depression. Probiotics are found to make vaginal flora friendlier to beneficial bacteria, and enhance anti-inflammatory or reduce pro-inflammatory cytokines. Researchers also found probiotics may decrease eczema in breastfed infants and prevent allergic reactions. 

Probiotics are a relatively inexpensive and easy intervention. You already take a daily prenatal vitamin, so adding a probiotic wouldn’t be a significant inconvenience. Other than the cost, the only downside is that common side effects of probiotics are initial digestive issues, like gas, bloating, or constipation. One way to limit the side effects is to start with a lower dose of probiotics and gradually increase to the suggested daily amount. Many probiotic supplements recommend starting with a lower dose and gradually increasing it over time. Everyone’s gut microbiome is different. If you take a probiotic that has bothersome side effects, you can always stop taking it.

Click here for my favorite probiotic. I chose this one based on many factors, including the strains of probiotics included in it, the brand, and the price. This probiotic includes all of the strains of bacteria that are evidence-based to help things like constipation and possibly lower the risk for group B strep. If you would like a probiotic marketed specifically for pregnancy, this one is a great option. AG1 is also a fantastic source of pre and probiotics. As with any supplement, please consult your doctor or midwife. For more in-depth information on probiotics, see this episode.

Future Vaccine

In the future, a vaccine may be available to prevent GBS colonization. The biotech company MinervaX and the pharmaceutical company Pfizer are each developing a GBS vaccine for pregnant women currently in phase II trials. We will need to wait for the results of ongoing trials and regulatory approval to determine how a GBS vaccine might influence prenatal care and recommendations for testing and treatment. If a GBS vaccine is approved and universally recommended for pregnant women, this will be the 5^th vaccine recommended during pregnancy. The currently recommended vaccines during pregnancy in the United States include Tdap, COVID, influenza, and RSV.

Talking to Your Doctor or Midwife

Please discuss any questions or concerns with your doctor or midwife. Your care provider is an excellent resource for answering any questions about the group B strep test, understanding your results, and knowing how to proceed in the event you test positive.

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