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Group B strep test

4-minute read

If you’re pregnant, one of the routine antenatal tests you’ll probably have is for group B streptococcus (also known as ‘group B strep’ or ‘GBS’). This page explains what the test is for, which babies are at risk of infection and how group B strep is treated.

What is group B strep?

Group B strep is a type of bacteria commonly found in the intestines, rectum, urethra or vagina. Many women have it, and it usually causes no health concerns or symptoms. Some women infected with group B strep get a urinary tract infection, and it increases the risk of a miscarriage only very slightly.

The main concern is passing it on to the baby before or during a vaginal birth. While many babies who get infected stay healthy, a small proportion (about 1 in 200) become very sick within the first few days of life. They get serious infections such as pneumonia and blood poisoning, or meningitis, which is an infection around the brain.

These seriously ill babies are treated with intravenous antibiotics. Most recover with no side-effects, but some don't survive.

Group B strep test

The group B strep test is a routine antenatal test. It involves taking a swab of the inside of the vagina. Your doctor or midwife will do this, or you may be able to do it yourself. The swab is taken at 35 to 37 weeks' pregnancy. Tests done earlier in your pregnancy aren't a good guide to your condition at birth because the bacteria can come and go.

Who has the group B strep test?

Some hospital and doctors recommend that every pregnant woman has a group B strep test.

But others ask only some women to have the test if they are at higher-than-average risk because they go into labour early, or their waters break early.

Even if you're planning a caesarean, you can think about having the test in case your waters break early or you go into labour unexpectedly early.

Talk about the risks and benefits of the group B strep test with your midwife or doctor.

Risk factors for infection with group B strep

Babies are more likely to be infected with group B strep if:

  • they are born, or your waters break, before 37 weeks
  • you give birth more than 18 hours after your waters break
  • you have a fever (over 38°C) while in labour
  • you've had a previous child with severe group B strep infection
  • a urine test during pregnancy detected group B strep
  • a swab taken no more than 5 weeks earlier detected group B strep

Preventing group B strep from infecting your baby

If your group B test at 35 to 37 weeks was positive, or if you have the risk factors listed above, your doctor or midwife will probably recommend that you have intravenous antibiotics when your waters break or labour starts.

Often this is penicillin, but alternatives are available if you are allergic to penicillin. The antibiotics pass through the placenta and into the baby. This greatly reduces your baby's chances of becoming ill.

There's no need for antibiotics before a planned caesarean because the baby will not pass through the vagina. You can't pass the bacteria to your baby through the placenta.

After the birth

If you had antibiotics during labour, your baby will be monitored for the first couple of days for signs of infection. The chance of your baby being infected is low.

Late-onset group B strep

There is a very small group of babies that develop group B strep infection weeks or months after birth (called ‘late onset group B strep'). They become seriously ill quite quickly, with fevers and vomiting and difficulty feeding. Unfortunately, the group B strep test and use of antibiotics doesn’t prevent a baby developing late-onset group B strep infection.

If your child's condition deteriorates quickly, call triple zero (000) and ask for an ambulance, or go to the nearest hospital emergency department.

When to seek help

If you want more information about group B strep, or if you have any concerns about your baby, call Pregnancy, Birth and Baby on 1800 882 436 to speak to a maternal child health nurse, or talk to your midwife or doctor.

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Last reviewed: May 2020

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