Streptococcus Agalactiae

If you are pregnant or planning to be, this is one of the few tests where a positive result directly changes what happens in the delivery room. Group B Streptococcus lives quietly in the vagina or rectum of roughly one in five women, causing no symptoms, but it is the leading bacterial cause of severe newborn infection when passed during birth.

A swab around 36 0/7 to 37 6/7 weeks of pregnancy tells your care team whether you need antibiotics during labor. That single decision is what stands between a healthy delivery and the risk of newborn sepsis, pneumonia, or meningitis in the first days of life.

What This Swab Actually Detects

GBS (group B Streptococcus, also called Streptococcus agalactiae) is a Gram-positive bacterium that normally lives in the gut and can spread to the vagina. It is not a sexually transmitted infection and it is not something you did wrong. It is part of the normal bacterial mix in many healthy adults, and most carriers never know they have it.

The swab is a presence-or-absence test. A positive result means the bacterium grew from your sample at the time of testing. It does not mean you are sick. It signals that, during birth, your baby would be exposed to GBS in the birth canal, which is the main route to early newborn infection.

How Common Is It

Colonization is common and varies by region. Reported rates from rectovaginal sampling in pregnant women generally range from about 9 to 33 percent in single-country studies, with an adjusted global average around 18 percent. Examples include 24.5 percent in Tanzania, 19.5 percent in Vietnam, 13 to 24 percent in Ethiopian sampling, and about 9 percent in Argentina. A large multi-country study across Africa and South Asia found 24.1 percent colonization at labor.

Translation: between roughly one in ten and one in three pregnant women test positive, depending on the population. Carriage itself is not a sign of poor health. What matters is knowing your status before labor.

Why It Matters for Your Baby

Maternal colonization is the main pathway to early-onset disease in newborns, which includes sepsis and pneumonia in the first days of life. There is also late-onset disease, which can include meningitis in the following weeks. Without intrapartum antibiotics, about 1 to 2 percent of babies born to colonized mothers develop early-onset disease, and rates are higher in the presence of additional risk factors.

Globally, maternal GBS colonization contributes to substantial numbers of infant infections, stillbirths, and children who survive sepsis or meningitis with moderate or severe neurodevelopmental impairment. A meta-analysis of universal screening versus risk-based protocols found that screening-based strategies are associated with a lower rate of early-onset GBS sepsis in newborns compared with risk-based approaches alone.

Why It Matters for You

GBS colonization is usually silent in the mother. Older studies linked it to infection of the membranes around the baby (chorioamnionitis), uterine infection after delivery, post-cesarean wound infection, and urinary tract infection. A study of 1,746 deliveries found that GBS colonization at the time of delivery was associated with higher rates of maternal infection around birth. However, more recent and larger studies in the era of routine intrapartum antibiotic prophylaxis show the opposite pattern for chorioamnionitis: a secondary analysis of 170,804 deliveries found GBS colonization was associated with lower odds of chorioamnionitis (adjusted odds ratio 0.89), and a multicenter cohort of 155,255 deliveries confirmed GBS-positive patients were about 21 percent less likely to be diagnosed with chorioamnionitis. The historical association appears to be reversed when colonized women receive antibiotics during labor.

Late-pregnancy colonization is also linked to broader changes in the vaginal microbiome, including reduced protective Lactobacillus crispatus and increased Lactobacillus iners, along with higher markers of inflammation such as neutrophils, C-reactive protein (CRP), and interleukin-6 (IL-6, an inflammation signal). These shifts are associated with conditions like premature rupture of membranes, preterm delivery, and adverse pregnancy outcomes.

Sample Site Matters: Why Rectovaginal Beats Vaginal Alone

GBS lives in the gut and migrates to the vagina, so a swab that samples only the vagina can miss carriers. In one comparison, combined rectovaginal swabbing detected 100 percent of carriers, while vaginal-only sampling caught 50 percent and rectal-only caught 82 percent. The standard recommendation is therefore a combined lower vaginal plus rectal swab. If your test uses only a vaginal sample, a negative result is less reassuring than a negative rectovaginal one.

How Accurate Is the Test

Enriched culture is the reference standard. Because positive growth is highly specific, a positive culture is almost always real, but culture can miss some low-level carriers that molecular methods pick up. In a large meta-analysis, real-time PCR had pooled sensitivity of about 96 percent and specificity of about 98 percent compared with enriched culture. One newer assay (Xpert GBS LB XC) reported about 99 percent sensitivity and 99 percent specificity against a composite comparator, and a recent algorithm study found that broth-enriched molecular testing detected about a third more positives than culture alone.

Source: Peng et al. 2025 meta-analysis of real-time PCR; Thwe et al. 2022 (Xpert GBS LB XC); Villa et al. 2025 (broth-enriched NAAT algorithm); Curry et al. 2018 (LAMP vs enriched culture).

What this means for you: if speed matters (such as during labor with unknown status), a rapid molecular test can quickly flag colonization. For routine third-trimester screening, an enriched rectovaginal culture remains the standard. Combining sample sites and using enriched media is more important than which specific method is used.

Higher Risk Groups Within Pregnancy

Some groups have notably higher carriage. In a Tanzanian study, GBS colonization was 63.1 percent in HIV-infected pregnant women versus 18.9 percent in HIV-negative women, with an adjusted odds ratio of about 7.3 (meaning HIV-infected women in this study were about 7 times more likely to be colonized). This is a single regional study and should not be read as a universal rate, but it points to higher risk in this group. Other factors associated with colonization include premature rupture of membranes, preterm delivery, gestational diabetes, abnormal vaginal discharge, and concurrent fungal colonization. Carriage rates also rise with diabetes during pregnancy, which is linked to higher rates of GBS infection.

Why One Result Is Not the Whole Picture

GBS colonization can come and go. The current evidence treats the swab as a presence-or-absence snapshot, not a stable lifelong number. That is exactly why timing matters: a single test near the end of pregnancy is more relevant to what happens at delivery than a test done months earlier, because colonization can change between trimesters.

Practical cadence for pregnancy: get the standard combined rectovaginal swab at 36 0/7 to 37 6/7 weeks. If labor starts before that screen, or if your status is unknown, a rapid intrapartum molecular test can guide antibiotic decisions in real time. If you had a previous baby with GBS disease, or GBS bacteria in your urine during this pregnancy, antibiotics during labor are recommended regardless of the swab.

When Results Can Be Misleading

• Sampling site: a vaginal-only swab misses about half of carriers compared with a combined rectovaginal swab. If you got a negative result from a vaginal-only sample, that result is less reliable than a combined sample.
• Media used: standard blood agar can miss non-hemolytic GBS strains. Chromogenic media and enrichment broths detect more carriers, so the lab's method affects whether a borderline carrier shows up.
• Timing: colonization status can shift across pregnancy. A swab done well before 36 weeks may not match your status at delivery.
• Recent antibiotic use: in a study of 668 pregnant women, recent oral antibiotics for unrelated infections did not significantly change vaginal GBS colonization rates. So antibiotics for something else are unlikely to falsely clear a true GBS carrier, but extended courses near the time of testing can complicate interpretation.

What to Do With an Out-of-Pattern Result

A positive GBS swab is not a diagnosis you need to fix in advance. It is a flag that you should receive intrapartum antibiotics (typically given through a vein during labor) to substantially reduce the chance of passing GBS to your baby. The decision pathway is direct: confirm with your obstetric team that you are flagged as GBS positive in your delivery plan, ensure your hospital or birth center is aware of any penicillin allergy (this changes which antibiotic is used), and make sure you arrive early enough in labor for antibiotics to be given for at least four hours before delivery when possible. One exception: if you are scheduled for a prelabor cesarean delivery with intact membranes, current ACOG guidance is that intrapartum antibiotics for GBS are not needed.

If you test positive, ask whether susceptibility testing should be done on your sample. Resistance to macrolides (a class that includes erythromycin) and clindamycin is common in some regions, and this matters if you are penicillin allergic. If you are HIV-positive, have diabetes, or have a history of preterm labor, raise GBS testing earlier with your care team, since these groups have higher colonization rates and benefit from clear planning.

Beyond Pregnancy

GBS can cause invasive disease in non-pregnant adults, particularly older adults and those with diabetes or immunosuppression, but routine vaginal GBS screening outside of pregnancy is not supported by current evidence to add value over standard clinical care. The strongest case for this test remains late pregnancy.