Group B Strep Test in Pregnancy: Screening, Labour Antibiotics and What Changes by Country

A group B strep test in pregnancy checks whether you carry Streptococcus agalactiae (group B Streptococcus, or GBS) in the vagina or rectum. Carrying GBS is common and not an illness in adults, but around the time of birth a small number of babies develop serious early-onset infection. How and when you are tested depends heavily on where you live: the United States recommends universal screening near 37 weeks, while the United Kingdom does not routinely screen every pregnant person.

This guide is general information, not medical advice. Only your midwife, obstetrician or family doctor can interpret your swab or urine results and plan antibiotics, birth place and newborn monitoring for your situation. If you are in labour, your waters have broken, or your newborn seems unwell, contact your maternity or emergency line now rather than waiting to finish reading.

Third-trimester planning overlaps with GBS timing; our pregnancy week by week guide and pregnancy week calculator can help you see when 36–37 weeks falls on your calendar. Pack birth preferences and contact numbers using the hospital bag checklist.

TL;DR: group B strep in pregnancy

Question	Practical answer
What is GBS?	Normal bacteria carried by many adults; can pass to the baby around birth
US screening	Vaginal–rectal swab at 36 or 37 weeks each pregnancy (CDC)
UK screening	Not offered routinely to all; care if GBS found on NHS tests or prior affected baby (NHS)
If swab positive	IV antibiotics in labour, not antibiotics before labour starts
Risk without labour antibiotics	CDC examples: about 1 in 200 early-onset disease if positive and untreated vs about 1 in 4,000 if positive with IV antibiotics in labour
UK background risk	NHS cites about 1 in 1,750 pregnancies where GBS spreads to the baby in labour
Newborn red flags	Breathing problems, floppiness, poor feeding, temperature issues — urgent assessment

What is group B strep?

Group B strep is one of many bacteria that can live in the intestines, vagina and rectum without causing symptoms. RCOG patient information states that in the UK about 2–4 in 10 women carry GBS at some point; it is not a sexually transmitted infection and carrying it is not harmful to you.

The concern is timing: most babies who develop early-onset GBS disease in the first week of life are exposed during labour or birth. Infection can present as sepsis, pneumonia or meningitis. CDC prevention guidance stresses that antibiotics given during labour to people at increased risk are the main strategy proven to reduce early-onset disease. Late-onset GBS (after the first week, up to several months) is less common and is not prevented by labour antibiotics alone.

Numbers help frame risk without frightening: the NHS estimates about 1 in 1,750 newborn GBS infections in UK pregnancies overall, while RCOG cites around 1 in 1,750 diagnosed with early-onset infection in the UK and Ireland, with most babies recovering when infection is treated promptly.

What does the group B strep test involve?

The standard screening culture is a vaginal–rectal swab collected with a sterile swab. CDC testing guidance describes it as simple and not painful. The sample goes to a laboratory; results are usually reported as positive or negative for GBS colonisation.

GBS can also appear on a midstream urine culture during pregnancy. That finding matters clinically: a urinary tract infection with GBS should be treated with antibiotics appropriate for the infection, and RCOG and NHS guidance both say you should also be offered IV antibiotics in labour to protect the newborn.

Status can change between pregnancies and even within pregnancy because GBS comes and goes in the body. That is why US guidance screens in each pregnancy near term, not only once in a lifetime.

When is the group B strep test done?

United States

The CDC recommends that healthcare providers screen all pregnant women during the 36th or 37th week of each pregnancy, following American College of Obstetricians and Gynecologists (ACOG) and American College of Nurse-Midwives (ACNM) timing. Screening is advised even when a cesarean birth is planned, because plans can change if labour starts early.

United Kingdom and Ireland

Screening for GBS is not routinely offered to every pregnant woman in the UK. The UK National Screening Committee position, summarised by RCOG, explains that universal late swabs cannot accurately predict which babies will become infected, a negative swab does not guarantee you are GBS-free at birth, and many affected babies are born preterm before typical screening weeks.

GBS may still be identified when you have vaginal swabs, rectal swabs or urine tests for other reasons. The NHS group B strep page says you can discuss risks and private testing with your midwife or GP if you are worried. RCOG notes enriched culture medium (ECM) swabs around 35–37 weeks may be used in specific situations, such as GBS in a previous pregnancy when the baby was unaffected, to help decide on labour antibiotics.

Why the difference matters for readers

If you moved countries, switched providers, or read US forums while under NHS care, you may hear conflicting advice. Neither approach claims GBS is harmless; they differ on population screening versus targeted prevention when GBS is known or other intrapartum risks appear. Bring your history (prior GBS-positive swab, GBS in urine, previous baby with GBS disease) to every booking.

What if my group B strep test is positive?

A positive term swab means you are colonised, not that you have a disease requiring treatment today. CDC prevention guidance is explicit that antibiotics before labour begins do not work for this purpose because GBS can grow back quickly, and oral antibiotics do not prevent early-onset newborn disease.

Instead, clinicians offer IV antibiotics during labour, typically beta-lactams such as penicillin or ampicillin when not contraindicated. CDC patient education uses illustrative odds: a woman who tests positive and receives IV antibiotics in labour might face about a 1 in 4,000 chance of early-onset GBS in the baby, versus about 1 in 200 if positive and labour proceeds without those antibiotics (CDC prevention page examples).

RCOG states that for carriers, labour IV antibiotics reduce risk from about 1 in 400 to about 1 in 4,000 for early-onset infection. Penicillin allergy should be documented so an alternative regimen can be planned.

Cesarean birth and GBS

If you have a planned cesarean before labour starts and membranes are intact, you generally do not need intrapartum antibiotics solely to prevent GBS in your baby—even when a swab was positive—per RCOG patient information and US neonatal prevention guidance. If you are GBS-positive and labour begins or waters break before cesarean, the plan shifts toward the same intrapartum antibiotic approach as vaginal birth. Your surgical team makes that call.

Waters broken and induction

RCOG advises that if membranes rupture after 37 weeks and you are known to carry GBS, you may be offered induction of labour to limit time exposed to GBS before birth, plus IV antibiotics. NHS infection-in-pregnancy guidance notes higher risk when waters break more than 24 hours before birth. Use the labor signs checker only as a reminder to call your team when labour may be starting — not to delay reporting broken waters.

Who gets labour antibiotics without a routine positive swab?

Even without a known positive culture, clinicians may offer IV antibiotics in labour when other risks stack up. Examples aligned with CDC risk factors and NHS pregnancy infection guidance include:

• Previous baby with GBS disease — antibiotics in labour in subsequent pregnancies (NHS and RCOG).
• GBS bacteriuria at any point in the current pregnancy — treat the urine infection and plan labour IV antibiotics.
• Unknown GBS status plus preterm labour, prolonged rupture of membranes (often 18 hours or more in US summaries, 24 hours in UK materials), or fever in labour.
• Preterm labour — RCOG recommends considering antibiotics even if GBS status is unknown.

Your birth plan should note allergies, especially penicillin allergy, so alternatives can be ready. IV antibiotics during labour are generally compatible with mobility and water birth when your unit supports it; RCOG says women should still be able to move around in many settings.

What does not prevent newborn GBS disease?

CDC prevention guidance lists strategies that are not effective for preventing early-onset GBS in babies:

• Oral antibiotics for the pregnant person outside labour
• Antibiotics given before labour starts to clear colonisation
• Vaginal chlorhexidine washes

Knowing this helps you interpret online “natural” protocols that lack official backing.

How will my baby be monitored after birth?

Monitoring depends on gestation, whether you received adequate intrapartum antibiotics, and other risks.

RCOG outlines that if you are at term and received IV antibiotics at least four hours before birth, your baby may need no special GBS monitoring. If the baby is at higher risk and antibiotics were not given at least four hours before birth, close monitoring for at least 12 hours is typical — heart rate, temperature, breathing and feeding checks.

If you previously had a baby with GBS infection, RCOG advises monitoring for 12 hours even when labour antibiotics were given.

Most babies who receive timely labour antibiotics do not need treatment. Babies who develop suspected infection need hospital antibiotics promptly; breast feeding remains appropriate when clinicians say it is safe (RCOG).

Our complete newborn care guide covers routine first-week checks and feeding patterns alongside infection awareness.

When to call your midwife or doctor

During pregnancy or labour

Contact your maternity team promptly if:

• You test positive for GBS or have GBS in urine — ask for a written labour plan and when to come in if waters break.
• Your waters break (clear or unclear timing) — GBS protocols often depend on hours since rupture.
• You develop fever, flu-like illness or urinary symptoms — infection in pregnancy needs assessment; see pregnancy warning signs.
• You go into labour before 37 weeks — preterm birth raises GBS-related risk and changes monitoring.

For your newborn

NHS guidance lists baby symptoms that need urgent help, including:

• Grunting, moaning or working hard to breathe
• Very sleepy or unresponsive behaviour
• Poor feeding or vomiting feeds
• High or low temperature, or skin feeling too hot or cold
• Floppiness or unusual crying
• Colour changes or blotchy skin
• Fast or slow heart rate or breathing

The NHS advises call 999 or go to emergency care if you think your baby has a GBS infection. In the US, use 911 or your local emergency number for the same concerns. Mention GBS exposure if known.

Late GBS can appear up to about three months; labour antibiotics do not prevent late-onset disease. Any sudden deterioration in a young infant warrants immediate clinical assessment.

What you can do now

1. Ask your provider which GBS policy applies to you (universal 36–37 week swab vs risk-based UK care) and whether your hospital uses enriched culture methods for private tests.
2. Record prior pregnancies — GBS status, urinary results, and whether any baby had GBS sepsis or meningitis.
3. Document penicillin allergy clearly in your notes and birth plan; ask what antibiotic substitute would be used.
4. Plan birth location with RCOG’s point in mind: home or midwife-led units may not always offer IV antibiotics — discuss transfer pathways if you are GBS-positive.
5. Know when to call once waters break — do not wait for contractions if your team told you to present for antibiotics.
6. Learn newborn warning signs and post emergency numbers where birth partners can see them.
7. Pair this guide with newborn preventive decisions such as the vitamin K shot and hepatitis B birth dose so you are not making each choice in isolation during exhaustion after birth.
8. Use the hospital bag checklist to include allergy lists, swab printouts and out-of-hours contact cards.

Sources and related reading

Official and primary sources:

• U.S. Centers for Disease Control and Prevention: Screening for group B strep bacteria (updated May 1, 2025).
• U.S. Centers for Disease Control and Prevention: Preventing group B strep disease in newborns.
• U.S. Centers for Disease Control and Prevention: Risk factors for group B strep disease.
• NHS: Group B strep.
• NHS: Infections in pregnancy that may affect your baby.
• Royal College of Obstetricians and Gynaecologists: Group B Streptococcus (GBS) in pregnancy and newborn babies (patient information based on Green-top Guideline No. 36).

Related Egspect guides and tools:

• Complete newborn care guide
• Pregnancy warning signs
• Pregnancy week by week trimester guide
• Vitamin K shot for newborns
• Hepatitis B birth dose for newborns
• Hospital bag checklist
• Pregnancy week calculator
• Labor signs checker

This guide is general information, not medical advice. Facts were accurate as of November 8, 2026; guidance changes, so check with your care team.