Lab result explainedInflammationLast updated November 11, 2026

GBS test in pregnancy: what positive and negative group B strep results at 36 weeks mean for you and your baby

Group B streptococcus test (GBS test) · Prenatal screening: rectovaginal swab culture at 36 0/7 to 37 6/7 weeks (also detected on urine culture) · reported in Reported as positive (GBS isolated) or negative (not isolated); urine cultures report colony-forming units per milliliter (CFU/mL)

The GBS test is a swab of the lower vagina and rectum, taken between 36 and 37 weeks of pregnancy, that looks for group B streptococcus (Streptococcus agalactiae), a bacterium that 10 to 30 percent of healthy women carry in the gut and vagina without any symptoms. It is not an infection and not a sexually transmitted disease; it is part of normal flora in about one in four pregnant women. The test exists because a baby who passes through a GBS-colonized birth canal can, uncommonly, inhale or swallow the bacteria and develop a serious infection (sepsis, pneumonia or meningitis) in the first week of life.

The result is reported as positive or negative, usually within 24 to 72 hours. A positive result means you will be offered intravenous antibiotics, typically penicillin, once labor starts or your water breaks, given at least four hours before delivery if possible. That single intervention cuts the baby's risk of early-onset GBS disease from roughly 1 in 200 to about 1 in 4,000, and is why US rates have fallen by more than 80 percent since screening began in the 1990s. A negative result means no antibiotics are needed for GBS, as long as you deliver within five weeks of the swab.

Carriage comes and goes, so a positive test in one pregnancy does not guarantee a positive test in the next, and a woman can be negative at 36 weeks and positive at delivery in a small number of cases. The screen is timed at 36 to 37 weeks because that is late enough to predict status at an average-timed birth and early enough to cover most spontaneous labors. Women who had GBS in their urine at any point in the pregnancy, or who previously had a baby with GBS disease, are treated in labor without needing the swab.

GBS test at a glance

What it is: a culture of a vaginal and rectal swab for group B streptococcus, done at 36 0/7 to 37 6/7 weeks of pregnancy; results take about 2 days.
Positive (10–30% of women): you carry GBS; plan for IV antibiotics (usually penicillin) in labor, started at least 4 hours before delivery when possible. The baby's risk falls from about 1 in 200 to 1 in 4,000.
Negative: no GBS antibiotics needed if you deliver within 5 weeks of the swab; a longer gap or preterm labor triggers a retest or risk-based treatment.
GBS in urine at any time in pregnancy, or a previous baby with GBS disease, means antibiotics in labor regardless of the swab.
A planned cesarean before labor with intact membranes does not need GBS antibiotics even if the test is positive.
Status unknown when labor starts: antibiotics are given if you are under 37 weeks, your water has been broken 18 hours or more, or you have a fever of 100.4°F (38°C) or higher.

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What GBS test measures

Group B streptococcus lives in the lower intestine and spreads to the vagina, so the test samples both: a swab is passed over the lower third of the vagina and then through the anal sphincter, in that order, without a speculum. Self-collection after instruction gives results as accurate as collection by a clinician. The swab goes into an enrichment broth that selectively grows streptococci for 18 to 24 hours, then onto agar or into a molecular assay; enrichment is what makes the test sensitive, and skipping it (as some rapid tests do) misses a fifth or more of carriers.

The report says GBS 'isolated' or 'positive' when the organism grows, and 'not isolated' or 'negative' when it does not. If you have reported a penicillin allergy, the lab also tests the strain against clindamycin, because 20 to 40 percent of GBS isolates in the US are now resistant to it, and the result determines which alternative antibiotic can be used in labor. Labs are asked to test for 'inducible' clindamycin resistance as well, so a strain that looks sensitive on first pass but switches on resistance during treatment is caught.

A separate route to the same information is the urine culture done at the first prenatal visit. GBS in the urine at any concentration means heavy colonization, which is itself an indication for antibiotics in labor, and at 100,000 colony-forming units per milliliter or more it is treated as a urinary tract infection at the time it is found. Newer rapid nucleic acid tests can give an answer in one to two hours during labor for women whose status is unknown; they are useful but less sensitive than the enriched culture, and a negative rapid test does not override risk factors.

Normal range for GBS test

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (Reported as positive (GBS isolated) or negative (not isolated); urine cultures report colony-forming units per milliliter (CFU/mL))	Notes
Negative (GBS not isolated)	No growth from the rectovaginal swab	No GBS antibiotics in labor unless there is GBS bacteriuria this pregnancy, a previous GBS-affected baby, or you deliver more than 5 weeks after the swab. Reported in about 70 to 90% of women.
Positive (GBS isolated)	Group B streptococcus grown from the swab	Found in 10 to 30% of pregnant women. Intravenous antibiotics in labor or after membrane rupture; not needed for a pre-labor cesarean with intact membranes.
GBS bacteriuria, low count	Under 100,000 CFU/mL on any urine culture this pregnancy	Marks heavy colonization. No treatment at the time, but antibiotics in labor without needing the 36-week swab.
GBS bacteriuria, high count	100,000 CFU/mL or more	Treated as a urinary tract infection when found, and again with antibiotics in labor.
Previous infant with GBS disease	No swab needed	Antibiotics in labor in every later pregnancy regardless of culture results.
Unknown status at labor	Treated by risk factors	Antibiotics if under 37 weeks, membranes ruptured 18 hours or more, temperature 100.4°F (38°C) or higher, a positive rapid NAAT, or GBS positive in a previous pregnancy (at the clinician's discretion).
Rapid intrapartum NAAT	Positive or negative within 1 to 2 hours	Sensitivity roughly 60 to 90% without enrichment. A positive result is acted on; a negative result does not cancel antibiotics in a woman with risk factors.
Early-onset GBS disease risk in the baby	About 1 in 200 if the mother is positive and untreated; about 1 in 4,000 with antibiotics in labor	US incidence overall is about 0.2 to 0.3 per 1,000 live births, down from 1.8 per 1,000 in the early 1990s.

Positive GBS test

A positive GBS test means the bacterium was growing in your vagina or rectum on the day of the swab. It is not an illness, it has no symptoms, it is not caught from a partner, and nothing you did caused it; it is part of the normal bacterial population in up to a third of women at any given time. It does, however, change the plan for labor, because the baby is exposed as it passes through the birth canal and after the membranes rupture. Without treatment about 50 percent of babies born to colonized mothers pick up the bacterium on their skin or in their airways, and 1 to 2 percent of those develop invasive early-onset disease, usually within the first 24 hours of life.

The treatment is intravenous antibiotics once labor begins or your water breaks, continued until delivery. Penicillin G is standard, ampicillin is an alternative, and cefazolin is used for women with a low-risk penicillin allergy; for a high-risk allergy the choice depends on the clindamycin susceptibility printed on your report, with vancomycin as the fallback. Antibiotics are most effective when started at least four hours before birth, which is why you will be told to come to the hospital promptly when contractions are regular or your membranes rupture rather than laboring at home for hours. Oral antibiotics before labor do not help, because the bacteria recolonize within days.

A positive result does not require a cesarean, and a planned cesarean before labor with intact membranes does not require GBS antibiotics, because the baby is not exposed to the birth canal. Nor does the result affect breastfeeding, skin-to-skin contact or rooming in. After delivery the baby is observed for at least 36 to 48 hours; a well-appearing baby whose mother received adequate antibiotics usually needs no blood tests, while a baby whose mother did not get at least four hours of treatment is watched more closely.

Common causes

Normal gut and vaginal colonization
GBS lives in the lower bowel of 15 to 35 percent of adults of both sexes and intermittently spreads to the vagina. This is the explanation in essentially every pregnant woman who tests positive; it is not a sign of poor hygiene or infection.
Heavy colonization with GBS bacteriuria
GBS in the urine means the bacterial load is high enough to spill into the urinary tract. Any amount in pregnancy counts as positive for labor planning, and 100,000 CFU/mL or more is also treated as a urinary infection when found.
Diabetes and obesity
Both are associated with higher colonization rates in population studies, probably through changes in vaginal and gut flora; women with gestational or pre-existing diabetes test positive somewhat more often.
Positive in a previous pregnancy
About half of women who carried GBS in one pregnancy carry it in the next. ACOG allows clinicians to offer antibiotics on this basis when the current status is unknown at delivery.
Sexual activity and multiple partners
GBS is not a sexually transmitted infection, but carriage rates are modestly higher with frequent intercourse and new partners, likely from transfer of gut flora. Partners do not need testing or treatment.
Timing and technique of the swab
Carriage fluctuates week to week, so the same woman can be negative in one pregnancy and positive in another. A correctly taken swab samples the vagina first and then the rectum; a vaginal-only swab misses up to 40 percent of carriers, and intravaginal creams or douching in the 24 hours before the test can affect it.
Younger age, Black race and lower socioeconomic status
Epidemiological studies find somewhat higher colonization rates in these groups, which is part of why universal screening replaced risk-based screening in 2002.

Symptoms that can go with it

None in the mother; GBS carriage causes no discharge, itching, odor or pain
Occasionally a GBS urinary tract infection in pregnancy: burning, frequency, lower abdominal discomfort, cloudy urine
Rarely in the mother around delivery: fever, uterine tenderness or foul-smelling fluid (chorioamnionitis or endometritis)
In the newborn within the first 24 hours to 7 days: fast or labored breathing, grunting, flaring nostrils, pauses in breathing
Newborn: temperature instability (fever or low temperature), unusual sleepiness, limpness or irritability
Newborn: poor feeding, pale, gray or bluish color, a fast heart rate
Late-onset GBS disease (7 days to 3 months, not prevented by antibiotics in labor): fever, poor feeding, lethargy, a bulging soft spot or seizures

Negative GBS test

A negative GBS test means no group B streptococcus grew from the swab, which is the result in roughly three out of four women. You will not receive antibiotics for GBS in labor, and the baby's risk of early-onset GBS disease is very low. The result is valid for five weeks; if you reach 41 weeks or more after a swab taken at 36 weeks, or labor is induced later than expected, the test may be repeated, because colonization can appear in the interval.

A negative swab is not an absolute guarantee. Because carriage is intermittent and some swabs are taken imperfectly, about 4 to 8 percent of women who screen negative are colonized by the time they deliver, and a minority of early-onset GBS cases in the US now occur in babies of mothers who tested negative. This is why newborns are still observed for breathing difficulty, temperature changes and poor feeding in the first two days regardless of the mother's result, and why risk factors such as prolonged rupture of membranes or a fever in labor are managed on their own merits.

A negative test this pregnancy also says nothing about late-onset GBS disease, which occurs between 1 week and 3 months of age, is acquired after birth from caregivers or the environment, and is not prevented by antibiotics during labor. Knowing the signs (fever, poor feeding, unusual sleepiness, irritability, breathing changes) matters for every parent, positive or negative.

Common causes

Genuinely not colonized
Most negative results are simply correct: GBS was not present in the vagina or rectum at 36 to 37 weeks, and the chance that it appears before delivery is small.
Intermittent carriage
GBS comes and goes in the same woman over weeks. A culture captures one moment, which is why a result older than 5 weeks is no longer relied on and why a small share of negative women are positive at delivery.
Vaginal-only or shallow swab
Omitting the rectal sample reduces detection by up to 40 percent. If your swab did not include the rectum and you are near term, ask whether it should be repeated.
Recent antibiotics or vaginal products
An antibiotic course for another reason in the week before the swab, or antifungal creams and douching in the 24 hours before, can temporarily suppress growth and give a false negative.
Culture without enrichment broth
Direct plating or some rapid tests skip the 18 to 24 hour enrichment step and miss 20 to 50 percent of carriers. Standard prenatal cultures in the US use enrichment, but point-of-care tests in labor may not.

Symptoms that can go with it

None; a negative result causes no symptoms and needs no treatment
Newborn warning signs still apply: grunting, fast breathing, fever or low temperature, poor feeding, floppiness or unusual sleepiness in the first week
Late-onset signs at 1 week to 3 months: fever, irritability, poor feeding, lethargy, a bulging fontanelle

Read it alongside

Urine culture — GBS in urine at any point in pregnancy is an independent reason for antibiotics in labor, and 100,000 CFU/mL or more is treated as a urinary infection when found.
Bacteria, leukocytes and nitrite in urine — the dipstick findings that often prompt the urine culture; nitrite is usually negative with streptococci, so a clean dipstick does not exclude GBS bacteriuria.
Glucose challenge and glucose tolerance tests — gestational diabetes raises colonization rates slightly and is screened in the same trimester window.
Rubella IgG, hepatitis B surface antigen, HIV and RPR — the first-visit prenatal infection screens; together with GBS they define what is needed to protect the baby at delivery.
Maternal white blood cell count and CRP — drawn if there is fever in labor to look for chorioamnionitis; a high count with fever changes the antibiotic plan beyond GBS prophylaxis.
Newborn CBC, CRP and blood culture — ordered when a baby shows signs of infection or when the mother had a fever or inadequate prophylaxis; the blood culture is the definitive test for GBS disease.
Strep test (rapid strep A) — a different bacterium (group A streptococcus) that causes sore throat; a positive throat test has nothing to do with GBS carriage.
Clindamycin susceptibility — reported with a positive GBS culture only if you have a penicillin allergy; 'resistant' or 'inducible resistance' means vancomycin will be used instead.

What usually happens next

Positive: confirm the result is written in your chart and on any hospital pre-registration, tell the triage nurse when you arrive in labor, and plan to come in as soon as contractions are regular or your water breaks so the antibiotics have at least 4 hours to work.
Positive with a penicillin allergy: ask what kind of reaction you had (rash versus hives, swelling or breathing trouble) and whether your isolate was tested against clindamycin; the answers decide between cefazolin, clindamycin and vancomycin. Penicillin allergy testing in pregnancy is safe and often clears the label.
Negative: no GBS antibiotics unless you had GBS in your urine this pregnancy or a previous affected baby. If you go past 41 weeks or are induced more than 5 weeks after the swab, ask whether a repeat is needed.
Positive and planning a cesarean: antibiotics for GBS are not needed if the operation happens before labor with intact membranes, but you should still have the swab result on record in case labor starts first.
After the birth: the baby is observed for 36 to 48 hours; ask the pediatric team what signs to watch for at home for the first 3 months, since late-onset disease is not prevented by labor antibiotics.
Next pregnancy: expect to be screened again at 36 to 37 weeks; a previous positive swab is not by itself a reason for antibiotics, but a previous GBS-infected baby is.

Diet, lifestyle and natural levers with evidence

What can move GBS test once the cause is known. Nothing here replaces finding the cause.

Oral probiotics (Lactobacillus rhamnosus GR-1 and L. reuteri RC-14)

Preliminary evidence

A Taiwanese randomized trial of 99 GBS-positive women given these two strains daily from 35 to 37 weeks until delivery found 43 percent converted to GBS-negative versus 18 percent on placebo. A later Canadian double-blind trial starting earlier in pregnancy found no significant difference in colonization at 36 weeks, and a 2022 systematic review concluded the evidence is too inconsistent to recommend probiotics as a way to clear GBS. A later negative swab after probiotics does not cancel the plan: once the 36-to-37-week screen is positive, ACOG and the CDC still advise antibiotics in labor, and probiotic conversion is not an accepted reason to skip them.

Do not rely on a probiotic to replace antibiotics in labor. Probiotics are generally safe in pregnancy but discuss any supplement with your prenatal clinician.

Vaginal washes, garlic, vinegar and other home remedies

Preliminary evidence

Chlorhexidine vaginal washes during labor have been tested in randomized trials and a Cochrane review: they reduce bacterial counts on the baby's skin but do not reduce early-onset GBS disease, so they are not recommended. Garlic, apple cider vinegar, tea tree oil and yogurt applied vaginally have laboratory activity against streptococci at best and no clinical trials showing they clear carriage or protect babies; douching can also disturb vaginal flora and is discouraged in pregnancy.

Nothing inserted vaginally in the 24 hours before the swab, and no douching at any point in pregnancy.

Getting the swab right

Strong evidence

The accuracy of the test depends more on collection than on anything else. Sampling the lower vagina and then the rectum roughly doubles detection compared with a vaginal swab alone, self-collection after instruction is as reliable as clinician collection, and placing the swab in enrichment broth before plating raises sensitivity from around 50 to 60 percent to more than 90 percent. Avoid vaginal creams, lubricants and intercourse for 24 hours before the test, and tell the clinician about any recent antibiotics.

Blood sugar control and general measures

Preliminary evidence

Diabetes and obesity are associated with higher GBS colonization rates in cohort studies, so following your gestational diabetes plan is sensible, but no trial has shown that improving glucose control clears GBS. Normal hygiene (wiping front to back, cotton underwear) does not change carriage either way; GBS is gut flora, not something acquired from poor cleanliness.

When to see a clinician promptly

Your water breaks and you are GBS positive, or your status is unknown: go to the hospital or birth center promptly rather than waiting at home, so antibiotics can be started at least 4 hours before delivery.
Fever of 100.4°F (38°C) or higher during labor or in the first 2 weeks after delivery, with or without uterine pain or foul-smelling discharge: same-day assessment for chorioamnionitis or endometritis.
Your newborn is breathing fast (more than 60 breaths a minute), grunting, flaring the nostrils, has pauses in breathing, looks blue or gray, or is unusually floppy or hard to wake: call 911 or go to the emergency department.
Any fever of 100.4°F (38°C) or higher in a baby under 3 months, or a baby who is feeding poorly, vomiting, unusually irritable or sleepy: emergency evaluation the same day, since late-onset GBS infection is not prevented by antibiotics in labor.
Burning or frequency when urinating during pregnancy, so a urine culture can be sent; a GBS urinary infection needs treatment now and changes the plan for labor.
You are GBS positive with a serious penicillin allergy and no one has discussed which antibiotic you will receive: raise it at your next visit so susceptibility testing and the plan are in place before labor.

Frequently asked questions

What does GBS positive in pregnancy mean?

It means group B streptococcus, a bacterium that about 1 in 4 women normally carry in the gut and vagina, was found on your 36 to 37 week swab. It is not an infection, has no symptoms and is not sexually transmitted. It matters because the baby can pick it up during birth, so you will be given IV antibiotics (usually penicillin) once labor starts or your water breaks. With antibiotics the baby's risk of early-onset GBS disease drops from about 1 in 200 to about 1 in 4,000.

When is the GBS test done and how long do results take?

Between 36 weeks 0 days and 37 weeks 6 days, per the 2020 ACOG guidance (it used to be 35 to 37 weeks). A swab of the lower vagina and then the rectum is placed in enrichment broth and cultured, with results in about 24 to 72 hours. The result is considered valid for 5 weeks. Women with GBS in their urine earlier in pregnancy or a previous baby with GBS disease skip the swab and are treated in labor automatically.

Does the GBS swab hurt, and can I do it myself?

It is quick and usually painless, no speculum is needed, and the rectal part is a brief insertion just past the anal sphincter. Studies show that women who collect their own swab after instruction get results as accurate as clinician-collected swabs, so many practices offer self-collection. Avoid vaginal creams, lubricants, douching and intercourse for 24 hours beforehand.

Can I get rid of GBS before labor?

Not reliably. Oral antibiotics taken before labor clear GBS only temporarily, with recolonization within days, so they are not used. One small trial found that a specific two-strain probiotic converted 43 percent of positive women to negative versus 18 percent on placebo, but a later trial found no effect and reviews do not recommend it. Vaginal garlic, vinegar or chlorhexidine washes have not been shown to protect babies. A later negative swab does not undo a positive screen: ACOG still advises antibiotics in labor for anyone who tested positive at 36 to 37 weeks, and probiotic conversion is not an accepted exception.

Do I need antibiotics if I am GBS positive and having a planned C-section?

No, not for GBS, provided the cesarean happens before labor begins and before your membranes rupture, because the baby is not exposed to the birth canal. You will still receive the standard single-dose antibiotic given before any cesarean incision. If labor starts or your water breaks before the scheduled date, GBS antibiotics are added.

What if I am GBS negative but my water has been broken a long time?

A negative swab within the past 5 weeks means GBS antibiotics are not routinely given, even with prolonged rupture, though your care team will monitor your temperature and the baby's heart rate and may recommend induction. If the swab is more than 5 weeks old or was never done, antibiotics are given once membranes have been ruptured 18 hours, if you are under 37 weeks, or if you develop a fever of 100.4°F or higher.

I am allergic to penicillin. What will I be given for GBS?

It depends on the allergy. A mild, non-hives rash from long ago is low risk and cefazolin is used. Hives, swelling, wheeze or anaphylaxis is high risk; your GBS isolate is then tested against clindamycin, which is used if the strain is susceptible (20 to 40 percent are not), and vancomycin is used otherwise. Penicillin allergy testing during pregnancy is safe and removes the label in most people who have it.

Will GBS affect breastfeeding or my baby after birth?

No. You can breastfeed, hold your baby skin to skin and room in as usual. The baby is observed for 36 to 48 hours for breathing difficulty, temperature changes and poor feeding; if you received adequate antibiotics and the baby looks well, no blood tests are needed. Late-onset GBS disease (1 week to 3 months) is acquired after birth and is not prevented by labor antibiotics, so a fever of 100.4°F or more in a baby under 3 months always needs same-day emergency evaluation.

References

American College of Obstetricians and Gynecologists. Prevention of group B streptococcal early-onset disease in newborns: ACOG Committee Opinion No. 797. Obstetrics and Gynecology (2020)
Group B strep disease. Centers for Disease Control and Prevention (2024)View source
Strep B test. MedlinePlus, US National Library of Medicine (2023)View source
Verani JR, McGee L, Schrag SJ. Prevention of perinatal group B streptococcal disease: revised guidelines from CDC, 2010. MMWR Recommendations and Reports (2010)
Nanduri SA, Petit S, Smelser C, et al.. Epidemiology of invasive early-onset and late-onset group B streptococcal disease in the United States, 2006 to 2015: multistate laboratory and population-based surveillance. JAMA Pediatrics (2019)
Puopolo KM, Lynfield R, Cummings JJ; Committee on Fetus and Newborn, Committee on Infectious Diseases. Management of infants at risk for group B streptococcal disease. Pediatrics (2019)
Ohlsson A, Shah VS. Intrapartum antibiotics for known maternal group B streptococcal colonization. Cochrane Database of Systematic Reviews (2014)
Ohlsson A, Shah VS, Stade BC. Vaginal chlorhexidine during labour to prevent early-onset neonatal group B streptococcal infection. Cochrane Database of Systematic Reviews (2014)
Ho M, Chang YY, Chang WC, et al.. Oral Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14 to reduce group B streptococcus colonization in pregnant women: a randomized controlled trial. Taiwanese Journal of Obstetrics and Gynecology (2016)
Sharpe M, Shah V, Freire-Lizama T, et al.. Effectiveness of oral intake of Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14 on group B streptococcus colonization during pregnancy: a midwifery-led double-blind randomized controlled pilot trial. Journal of Maternal-Fetal and Neonatal Medicine (2021)
Hanson L, VandeVusse L, Malloy E, et al.. Probiotic interventions to reduce antepartum group B streptococcus colonization: a systematic review and meta-analysis. Midwifery (2022)
Group B strep disease. Mayo Clinic (2023)View source

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