Group B strep in pregnancy: screening, risks and antibiotics

What is group B strep?

Group B streptococcus (Streptococcus agalactiae, often shortened to GBS) is a bacterium that lives naturally in the bowel and, in some women, in the vagina. This is called carriage: the bacterium is present without causing illness or symptoms. Carriage can come and go, which explains why a swab can be positive in one pregnancy and negative in the next.

A worldwide meta-analysis estimates that about 18% of pregnant women are carriers. It is not a sexually transmitted infection, nor a sign of poor hygiene.

What are the risks of group B strep for the baby and the mother?

The main risk concerns the newborn. During labour or after the waters break, the bacterium can pass to the baby. Most often, it colonises the baby without causing illness. Rarely, it causes an early-onset newborn infection in the first days of life: blood infection (sepsis), pneumonia or meningitis. Group B strep remains one of the leading causes of serious infection in newborns.

In the mother, the bacterium can cause:

• urinary infections during pregnancy;
• infection of the membranes around the baby (chorioamnionitis), which can lead to premature birth;
• infection of the womb after the birth (endometritis), causing fever and pain. See fever after giving birth.

How is group B strep tested for in pregnancy?

In France, ANAES (now the French National Authority for Health, HAS) has recommended routine screening for all pregnant women between 34 and 38 weeks of pregnancy (weeks of amenorrhoea) since 2001. This timing gives a result that reflects the situation at the time of birth.

• The swab is taken with a long cotton bud passed at the entrance of the vagina and over the vulva. It is quick and painless.
• It is done by your doctor, your midwife or at the laboratory.
• The result is available within a few days. Keep it in your file and show it at the maternity unit.

Screening is not needed if an antibiotic will be given during labour anyway: the bacterium was found in your urine during pregnancy, or a previous baby had a group B strep infection.

Does group B strep need treating during pregnancy?

No. An antibiotic taken during pregnancy does not get rid of carriage for good: the bacterium often comes back before the birth. It therefore does not protect the baby and is not recommended for simple carriage without symptoms.

Treatment is, however, prescribed:

• for a urinary infection or bacteria in the urine (bacteriuria), according to the usual criteria; group B strep found in the urine also means an antibiotic during labour;
• if the waters break early, before term, when antibiotics are recommended to prevent infection;
• if there are signs of infection, based on medical advice.

When are antibiotics given during labour for group B strep?

The antibiotic is given by drip, as soon as labour starts or as soon as the waters break. It crosses the placenta and reduces the risk of early infection in the newborn, according to a Cochrane review whose trials carry a risk of bias. It is more effective when given at least four hours before the birth: tell the maternity unit about your positive result as soon as you arrive.

The usual antibiotic is a penicillin (amoxicillin or penicillin G). Mention any allergy: another antibiotic will be chosen based on the sensitivity test done on the swab. After the birth, the paediatric team monitors the baby according to the HAS guidelines on newborns at risk of infection.

How is the newborn monitored after birth?

Not all babies are affected in the same way. The paediatric team takes into account your swab result, how long you received the antibiotic before the birth, the stage of pregnancy and any fever during labour.

• If the antibiotic was given at least four hours before the birth and the baby is well, clinical monitoring in the maternity unit is usually enough: temperature, breathing, colour, feeding.
• If the antibiotic was given too late, or if the mother had a fever or the baby was born prematurely, monitoring is closer and may include a blood test for the baby.
• Antibiotic treatment for the newborn is only started if there are signs of infection or abnormal test results.

The usual course of labour and birth is explained in our article on vaginal birth.

Which signs need urgent care at the maternity unit?

If your swab is positive, go to the maternity unit as soon as your waters break, even without contractions, or as soon as labour starts, so you receive the antibiotic in time. After the birth, seek help quickly if your baby has a fever or a low temperature, refuses to feed, is unusually floppy or sleepy, breathes fast or looks grey: in France, call 15.

Some signs should not wait for your next appointment. Go directly to your maternity unit's emergency department (open day and night), Maternité Sainte-Félicité or Clinique Sainte-Thérèse depending on where you are registered, if you have:

• any vaginal bleeding, even light;
• a leak of fluid, clear or coloured, from the vagina;
• regular or painful contractions before 37 weeks;
• fewer or no movements from the baby (once you usually feel them);
• a fever of 38 °C or more, shivering, or burning when passing urine with back pain;
• severe headaches, vision problems, pain under the right ribs or in the pit of the stomach, sudden swelling of the face or hands;
• severe or constant abdominal pain, a fall or a blow to the abdomen;
• intense itching of the palms and soles of the feet, especially at night.

If you feel faint, have heavy bleeding or very severe pain, call 15 (SAMU) or 112 in France, or your local emergency number.

Frequently asked questions

General information article: it does not replace a medical consultation. In an emergency, call 15 or 112 in France, or your local emergency number.