Group B Strep in pregnancy: what the test shows and what it means for labour

Pregnancy · Updated July 2026 · All articles

Group B Strep (GBS) is one of the topics that comes up repeatedly in pregnancy conversations and online forums, often with more alarm than the situation warrants for most people. The key things to understand are: carrying GBS is not an illness; it is extremely common; for the majority of GBS carriers the pregnancy and birth proceed without issue; and where there are specific risk factors, there are effective interventions. Here is a clear, grounded explanation.

What Group B Strep is

Group B Streptococcus is a bacterium that lives naturally in the bowel and genital tract of a significant proportion of the adult population. It is a commensal organism, meaning it normally lives in the body without causing any harm or symptoms. Most people who carry GBS are entirely unaware of it, have never been tested for it, and will never have a health problem related to it. Carriage is not an infection.

GBS becomes relevant in pregnancy for one specific reason: there is a small risk that during vaginal birth, the bacterium can be passed from the birthing parent's vagina or rectum to the newborn, and in a small number of cases it can cause serious infection in the baby. This neonatal GBS infection is uncommon but it can be severe when it occurs, which is why it receives careful clinical attention.

How common is GBS carriage?

Around 25 to 30 percent of adults in the UK carry GBS in the gut or vagina at any given time. Because carriage fluctuates, a person might test positive at one point in pregnancy and negative at another, or vice versa. This is one of the reasons why the UK's approach to GBS prevention focuses more on risk factors for neonatal infection than on universal carrier testing, though this is an area of ongoing clinical debate.

GBS is not sexually transmitted and is not related to hygiene. It is not contagious between adults in any meaningful sense. Telling someone you carry GBS has no implications for them unless they are pregnant or a healthcare provider caring for you in labour.

Testing in pregnancy: NHS versus private

In England, Wales and Northern Ireland, the NHS does not offer routine universal screening for GBS to all pregnant people. This is a policy decision based on cost-effectiveness modelling and concerns about the risks and practicalities of treating the large number of people who would test positive but whose babies are not at elevated risk. GBS is tested for on the NHS when it is found incidentally during another test (for example, in a urine culture), when you have had a previous baby affected by GBS, or when risk factors emerge during labour.

Private GBS testing is available and widely used. The most sensitive test is the enriched culture medium (ECM) swab, taken from the lower vagina and rectum at 35 to 37 weeks of pregnancy. This method is significantly more accurate than the standard culture used in some clinical settings. The Group B Strep Support charity (GBSS) provides information on how to access ECM testing in the UK. Testing at 35 to 37 weeks gives a result that is reasonably representative of carriage status at the time of birth.

What actually matters: risk factors for neonatal infection

The most important thing to understand about GBS in pregnancy is that carriage alone is not the primary driver of neonatal infection risk. The question is not simply "does this person carry GBS?" but "what risk factors are present that increase the likelihood of the baby becoming infected even if GBS is present?"

The RCOG identifies the following as key risk factors that should prompt consideration of intrapartum antibiotics, regardless of whether a formal GBS test has been done:

• A previous baby who developed GBS infection
• GBS identified in the urine during the current pregnancy (which indicates heavy carriage)
• Preterm birth (before 37 completed weeks)
• Prolonged rupture of membranes (waters broken for more than 18 hours before birth)
• Fever during labour above 38 degrees Celsius

If none of these risk factors are present and you test positive on a routine swab, your care team will discuss the options with you, but antibiotics during labour are not automatically indicated. The conversation is nuanced and your clinical team is the right source of personalised guidance.

Intrapartum antibiotics: what they do and how they are given

When intrapartum antibiotic prophylaxis (IAP) is indicated, you will be offered intravenous antibiotics during labour. The standard antibiotic is benzylpenicillin (penicillin G), given through a drip into a vein. If you have a penicillin allergy, alternatives such as clindamycin or vancomycin are used, depending on the sensitivity of the specific GBS strain found.

For IAP to be most effective at reducing the risk of early-onset GBS disease in the baby, the first dose should ideally be given at least four hours before birth. This means that notifying the labour ward of your GBS status as early as possible when labour begins is important, particularly if you are expecting a quick labour. The antibiotics are safe for you and for the baby during labour and do not cause the baby any harm.

It is worth noting that IAP prevents early-onset GBS disease (which occurs in the first week of life) but does not prevent late-onset GBS disease (which occurs between one week and three months of age). Late-onset GBS is thought to arise from community sources rather than from transmission during birth and is a different clinical scenario.

Signs of GBS infection in a newborn: what parents should know

Whether or not you received antibiotics in labour, it is important to know the warning signs of GBS infection in a newborn. Early-onset GBS disease typically presents within the first 24 hours of life, though it can occur up to seven days after birth.

Signs to watch for include: the baby being unusually sleepy or difficult to rouse; refusing to feed or feeding very poorly; grunting, rapid or laboured breathing, or unusual pauses in breathing; a high temperature (above 38 degrees) or an unusually low temperature; pale, mottled or blotchy skin; a high-pitched or unusual cry; and being floppy or unusually stiff. If your newborn shows any of these signs, seek medical attention immediately. GBS infection in newborns can progress rapidly and early treatment with antibiotics is highly effective when given promptly.

Reassurance for most GBS carriers

If you have tested positive for GBS and have none of the elevated risk factors listed above, it is worth hearing clearly: the overwhelming majority of GBS-positive people have uncomplicated pregnancies, labours and births, and their babies are born healthy. GBS carriage is not a diagnosis that requires ongoing management or treatment during pregnancy itself. It is information to be documented, shared with your birth team, and acted on only if specific risk factors are present during labour.

Feeling anxious about a positive GBS test is understandable, and it is entirely reasonable to discuss the options with your midwife or obstetrician. The Group B Strep Support charity is also a good resource for evidence-based information and support for people who are concerned.

Frequently asked questions

What is Group B Strep?

Group B Streptococcus (GBS) is a bacterium that lives harmlessly in the gut and vagina of around 25 to 30 percent of adults. Most people who carry it never know. It only becomes a concern in pregnancy because of the small risk of it passing to a newborn during labour and causing serious infection.

Does the NHS test all pregnant people for Group B Strep?

No. Routine universal screening for GBS is not currently offered by the NHS. Testing is carried out if you have specific risk factors or if GBS is found incidentally during another test. Private testing using an enriched culture medium (ECM) swab at 35 to 37 weeks is available and the most sensitive method of detection.

If I test positive for GBS does my baby definitely need antibiotics?

Testing positive for GBS means you carry the bacterium, not that your baby will necessarily be infected. The decision to offer intrapartum antibiotics depends on risk factors, including whether you have had a previous baby with GBS, are in preterm labour, have prolonged rupture of membranes, or have a fever in labour. Low-risk carriers are monitored but antibiotics are not always necessary.

What are the risk factors for neonatal GBS infection?

The main risk factors are: a previous baby affected by GBS; preterm birth (before 37 weeks); prolonged rupture of membranes (waters broken for more than 18 hours before birth); fever during labour (above 38 degrees); and GBS found in urine during the current pregnancy (which indicates heavy carriage). These factors guide who is offered intrapartum antibiotics.

What are intrapartum antibiotics and how are they given?

Intrapartum antibiotics for GBS prevention are usually intravenous penicillin (or clindamycin if you are allergic) given via a drip during labour. They are most effective when given at least four hours before birth. They significantly reduce the risk of the baby developing early-onset GBS infection.

What are the signs of GBS infection in a newborn?

Signs of GBS infection in a newborn include: being unusually sleepy or difficult to rouse; poor feeding; grunting, fast or laboured breathing; high or low temperature; pale or mottled skin; and an irritable or high-pitched cry. If your baby shows any of these signs in the first week of life, seek medical attention immediately.

This article is for general information only. Always consult your midwife, obstetrician, or GP for advice specific to your pregnancy.