The essentials

The group B strep (GBS) test is a vaginal and rectal swab done at 36 to 37 weeks of pregnancy. A positive result is common, about 1 in 4 pregnant people: it means you carry a usually harmless bacterium, not that you are sick. You receive IV antibiotics during labor, which make an infection in your baby very unlikely.

How do I read my group B strep test result?

The result is not a number: the lab says whether GBS grew from your swab or not. Here is a fictitious US report of a positive test.

Fictitious example of a lab report line (United States format)
TestResultReference rangeFlag
Specimen: vaginal-rectal swab, 36 weeks 2 daysReceived--
Group B Streptococcus (S. agalactiae) culture, enrichment brothPositiveNegativeA
Clindamycin susceptibility, GBS isolateResistantSusceptibleA
  1. Specimen line: one swab of the lower vagina, then the rectum. Sampling both sites finds GBS far more often than the vagina alone[3].
  2. "Enrichment broth": the swab is first grown in a special liquid. Without it, up to half of carriers can test falsely negative[3].
  3. "Positive", flag A: A means abnormal, not high or low. You are colonized with GBS; you are not ill[4].
  4. Susceptibility line: printed only when you have a severe penicillin allergy, so your team can choose the right backup antibiotic[3].

A GBS swab, a urine culture and a blood count on the same prenatal report? BloodSense reads your whole report line by line and helps you prepare your questions for your next visit.

Get my prenatal results explained

Why is the group B strep test done in pregnancy?

Group B streptococcus (Streptococcus agalactiae) lives in the gut and genital tract of many healthy adults. It is a different germ from group A strep, the cause of strep throat and scarlet fever. In a newborn, though, GBS is the leading cause of infection in the first days of life[1][2]. The swab is not part of the TORCH panel in pregnancy, a set of blood tests for toxoplasmosis, rubella, CMV and herpes antibodies.

About half of carriers pass the bacterium to their baby during labor or after the water breaks. Without antibiotics in labor, 1% to 2% of those babies develop early-onset GBS disease[1]: sepsis, pneumonia or meningitis[6]. Screening finds carriers so that they can be treated at the right moment. Since prevention began, early-onset disease in the United States fell from 1.7 cases per 1,000 live births in the early 1990s to 0.34-0.37 by the late 2000s[3].

What do positive and negative GBS results mean?

Each result type below is read differently. Your laboratory's wording and range prevail.

Group B strep results in pregnancy and what usually follows
ResultMeaningUsual next stepSource
Swab negative (36-37 weeks)No GBS foundNo GBS antibiotics in labor, whatever the risk factors (fever is still treated)CDC 2010[3]
Swab positiveColonization, without illnessIV antibiotics from the start of labor or when the water breaksACOG 2020[1]
GBS in urine, any trimesterHeavy colonizationTreatment now if it is an infection, and IV antibiotics in labor; no swab neededCDC 2010[3], March of Dimes[9]
Status unknown at laborNo result availableAntibiotics if birth before 37 weeks, water broken 18 hours or more, fever of 100.4 °F (38.0 °C) or more, or a positive rapid testCDC 2010[3]

A negative swab taken within 5 weeks of birth predicts the status at delivery well, in 95% to 98% of cases[3]. It is not a guarantee: the bacterium can come and go, so your baby is still watched after birth[3][8].

Why did my group B strep test come back positive?

You did nothing wrong. GBS is not caught from food, objects or sex[9]. From the most frequent to the rarest:

  • Common Harmless carriage: 10% to 30% of pregnant people in the United States carry GBS, often without ever knowing[3]; about 18% worldwide[10]. Carriage can be temporary, intermittent or lasting[3].
  • Possible GBS in the urine: found on a routine urine culture, it signals heavy colonization; with burning or fever it is a urinary tract infection treated by mouth right away[3][9].
  • Rare Infection in the mother: GBS can cause an infection of the uterus during or after birth, with fever[9].
  • Rare Infection in the baby: with antibiotics in labor, about 1 in 4,000 babies of carriers gets GBS disease, against 1 in 200 without[5].

Antibiotics taken before labor do not help: GBS grows back quickly[5][9]. That is why treatment waits for labor. It works best when started at least 4 hours before birth, but care is never delayed to reach that mark[1]. Penicillin or ampicillin is used most; about 1 in 10 people have mild side effects, and a severe allergic reaction occurs in about 1 in 10,000[5][9]. A planned cesarean before labor, with intact membranes, does not need GBS antibiotics[1].

The test does not protect against late-onset disease, which starts between 7 and 89 days of age[7]; no strategy prevents it yet[2].

How is the GBS swab done, and what can skew the result?

No fasting or preparation is needed. The swab takes under a minute and results usually come back in 1 to 2 days[9][12]. Check:

  • Timing: the swab is done between 36 weeks 0 days and 37 weeks 6 days, in every pregnancy[1][4]. A result older than 5 weeks becomes less reliable[3].
  • Both sites: the lower vagina, then the rectum. Your provider may let you swab yourself; follow the instructions exactly[3][9].
  • Even if a cesarean is planned, do the test: labor or a broken water can come first[4][12].
  • Tell your team about any penicillin allergy and what happened: rash, hives, swelling or breathing trouble. With a severe allergy, the lab tests which backup antibiotic works[3].
  • A delayed sample loses accuracy: it should reach the lab and be processed within 24 hours[3].
  • If GBS was found in your urine earlier in this pregnancy or a previous baby had GBS disease, say so: you get antibiotics in labor anyway[3].

If you go into labor early, before your result, you may be treated while waiting[3]. See also complete blood count and CRP, often ordered when a newborn is checked for infection.

When should I see a doctor about group B strep?

When to see a doctor

  • Emergency: call 911 if your baby has a fever or a low temperature, breathing trouble, a bluish color, feeds very poorly, is limp or hard to wake, or has a seizure, in the first weeks or months of life[6][8].
  • Today, without waiting if your water breaks or labor starts: go to your birth center and say you are GBS positive. The same day for fever, burning when you pee or back pain during pregnancy.
  • At your next appointment for a positive result without symptoms, to plan antibiotics in labor and check your allergy history.

Questions to ask your doctor

  1. When will I start antibiotics, and should I come in early once labor starts?
  2. Is my penicillin allergy real, and which antibiotic would I get?
  3. Does my planned cesarean change anything if my water breaks first?
  4. How long will my baby be watched after birth, and which signs should worry me at home?

Frequently asked questions

Does a positive GBS test mean I need a C-section?

No. A vaginal birth is the norm; antibiotics in labor protect the baby. A cesarean is chosen for other reasons[1][9].

Will my baby be tested after birth?

Usually not, if you had antibiotics in labor and your baby is well[9]. Babies at higher risk are watched in hospital for 36 to 48 hours, sometimes with blood tests and antibiotics until results come back[8].

Can I be GBS positive in one pregnancy and negative in the next?

Yes. Carriage comes and goes, which is why the test is repeated in every pregnancy[3][4]. A positive result in a past pregnancy is worth mentioning if your status is unknown at labor.

Is there a vaccine against group B strep?

Not yet. Vaccines for pregnant people are being studied; screening and antibiotics in labor remain the best protection[9].

  1. American College of Obstetricians and Gynecologists. Prevention of Group B Streptococcal Early-Onset Disease in Newborns: ACOG Committee Opinion, Number 797. Obstet Gynecol. 2020;135(2):e51-e72. doi:10.1097/AOG.0000000000003668 (accessed November 1, 2026)
  2. 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;144(2):e20191881. doi:10.1542/peds.2019-1881 (accessed November 1, 2026)
  3. Verani JR, McGee L, Schrag SJ. Prevention of perinatal group B streptococcal disease: revised guidelines from CDC, 2010. MMWR Recomm Rep. 2010;59(RR-10):1-36. cdc.gov (accessed November 1, 2026)
  4. Centers for Disease Control and Prevention. Group B Strep: Testing. cdc.gov (accessed November 1, 2026)
  5. Centers for Disease Control and Prevention. Preventing Group B Strep Disease. cdc.gov (accessed November 1, 2026)
  6. Centers for Disease Control and Prevention. Symptoms of Group B Strep Disease. cdc.gov (accessed November 1, 2026)
  7. Centers for Disease Control and Prevention. Clinical Overview of Group B Strep Disease. cdc.gov (accessed November 1, 2026)
  8. MedlinePlus Medical Encyclopedia. Group B streptococcal septicemia of the newborn. US National Library of Medicine. medlineplus.gov (accessed November 1, 2026)
  9. March of Dimes. Group B strep infection. marchofdimes.org (accessed November 1, 2026)
  10. Russell NJ, Seale AC, O'Driscoll M, et al. Maternal Colonization With Group B Streptococcus and Serotype Distribution Worldwide: Systematic Review and Meta-analyses. Clin Infect Dis. 2017;65(suppl_2):S100-S111. doi:10.1093/cid/cix658 (accessed November 1, 2026)
  11. HealthCare.gov. What Marketplace health insurance plans cover. healthcare.gov (accessed November 1, 2026)
  12. Cleveland Clinic. Group B Strep in Pregnancy: Test, Risks & Treatment. my.clevelandclinic.org (accessed November 1, 2026)

This page is general information. It does not replace the advice of a doctor or midwife, who interprets your results with your history, your pregnancy and your examination.