Group B strep test in pregnancy: when it is done and what results mean

Group B streptococcus (GBS) is one of the many bacteria that live in the body. It usually does not cause serious illness, and it is not a sexually transmitted infection. (ACOG) In the US, a swab test late in pregnancy is part of routine prenatal care; (ACOG CO 797, 2020) in the UK, the UK NSC does not recommend screening pregnant women for GBS. (UK NSC)

Why it is checked in pregnancy

In women, GBS is most often found in the vagina and rectum, so it can pass to the fetus during labor. This is rare: it happens to 1 or 2 babies out of 100 when the mother does not receive antibiotics during labor, and the chance is much lower when she does. About 1 in 4 pregnant women carry GBS. The risk of GBS disease is higher in babies born before 37 weeks. (ACOG)

Early-onset disease: a baby typically gets sick within 12 to 48 hours after birth or up to the first 7 days. It can cause meningitis, pneumonia or sepsis. (ACOG)
Late-onset disease: a baby gets sick between a week and a few months after birth, usually from contact with the mother after delivery if she is infected, but it can also come from other people who have GBS. (ACOG)

GBS septicemia in newborns now occurs less often, because there are methods to screen and treat pregnant women whose babies are at risk. (MedlinePlus, 2024)

When the test is done

ACOG says you should be screened for GBS as part of routine prenatal care. (ACOG) Its Committee Opinion 797 recommends universal screening between 36 0/7 and 37 6/7 weeks of pregnancy, and replaces the obstetric parts of the CDC's 2010 guidelines. (ACOG CO 797, 2020) It was published in February 2020 and is still current; a later interim update changed only the wording on allergy testing and dosing. (ACOG CO 797, 2020) Other checks across pregnancy: pregnancy screenings by week.

How the swab is taken

The test is a culture. A swab is used to take a sample from the vagina and rectum. (ACOG) ACOG lists correct specimen collection and processing among the key steps for preventing early-onset disease. (ACOG CO 797, 2020) Ask your provider who takes the swab and when to expect the result.

What the results mean, in general terms

Group B strep results and situations, and what the sources say they mean
Result or situation	What the sources say
Positive (GBS present)	Antibiotics are usually given through an IV line once labor has started, to help protect the fetus from being infected. The best time for treatment is during labor. This lowers the chance of early-onset disease but does not prevent late-onset disease. (ACOG)
Negative	A negative screen does not mean the baby has no risk, because the bacteria are not always detectable by screening the mother. (MedlinePlus, 2024)
GBS found in your urine	GBS bacteria in your urine at any point during pregnancy is one of the reasons antibiotics may be given during labor without testing first. (ACOG)
Not known when labor starts	Antibiotics may be given without a result if you have a fever, go into labor before 37 weeks, or it has been 18 hours or more since your water broke. (ACOG)

GBS usually does not cause serious illness, and although it is fairly common in pregnancy, very few babies get sick with GBS disease. (ACOG)

Planned cesarean birth: antibiotics for GBS are not needed during delivery if labor has not started and the amniotic sac has not ruptured. You should still be tested, because labor may happen before a cesarean birth. If the result is positive, the baby may need to be monitored for GBS disease after birth. (ACOG)

Your doctor decides what your result means for you and what happens during labor. This page does not give treatment advice. If a result worries you, see after an abnormal screening result for questions to ask.

Why it comes up again in a later pregnancy

Screening is part of routine prenatal care, and ACOG recommends it for everyone at 36 0/7 to 37 6/7 weeks. (ACOG) (ACOG CO 797, 2020)
A result can change: the UK NSC notes that a woman may have a positive result a few weeks before labour and a negative result when she gives birth. (UK NSC)
If your GBS status for this pregnancy is not known but you tested positive in a past pregnancy, antibiotics may be given during labor without testing. (ACOG)
If you had a previous child who had GBS disease, antibiotics may be given during labor without testing. (ACOG)

Signs to watch for in your baby

In newborns, the signs of meningitis can be hard to spot. ACOG says to contact your baby's health care professional right away if your baby has any signs or symptoms of disease, including lack of energy, irritability, poor feeding or high fever. (ACOG)

MedlinePlus says: (MedlinePlus, 2024)

This disease is usually diagnosed shortly after birth, often while the baby is still in the hospital.

However, if you have a newborn at home who shows symptoms of this condition, seek immediate emergency medical help or call 911 or the local emergency number.

Parents should watch for symptoms in their baby's first 6 weeks. The early stages of this disease can produce symptoms that are hard to spot.

The symptoms MedlinePlus lists include a blue appearance, breathing difficulties, an irregular or abnormal heart rate, lethargy, a pale appearance with cold skin, poor feeding, and an unstable body temperature (low or high). (MedlinePlus, 2024)

Where they differ:

Who is tested: in the US, ACOG recommends universal screening with a vaginal-rectal culture. (ACOG CO 797, 2020) In the UK, the UK NSC does not recommend screening pregnant women for GBS; its last review was in March 2017 and it will review the evidence again after the GBS3 trial. (UK NSC)
Why the UK says no: the test cannot accurately tell apart mothers whose babies are at risk from those who are not, so a large number of women would be offered antibiotics unnecessarily. (UK NSC, 2017)
Who is offered antibiotics in labour in the UK: NICE lists people in preterm labour; people with GBS found during the current pregnancy; people with GBS in a previous pregnancy who have not had a negative test at 35 to 37 weeks in this one; people who had a previous baby with an invasive GBS infection; and people with a clinical diagnosis of chorioamnionitis. (NICE NG195)
Timing: ACOG's Committee Opinion says 36 0/7 to 37 6/7 weeks and its FAQ says 36 to 38 weeks; (ACOG CO 797, 2020) (ACOG) MedlinePlus says 35 to 37 weeks. (MedlinePlus, 2024)

Questions people ask

Is group B strep a sexually transmitted infection?

No. GBS is one of the many bacteria that live in the body. It usually does not cause serious illness and it is not a sexually transmitted infection. (ACOG)

Is group B strep the same as strep throat?

No. Although the names are similar, GBS is different from group A streptococcus, the bacteria that causes strep throat. (ACOG)

When is the group B strep test done?

ACOG recommends universal screening between 36 0/7 and 37 6/7 weeks of pregnancy. Its patient FAQ says between 36 and 38 weeks. (ACOG CO 797, 2020) (ACOG)

How common is group B strep in pregnancy?

About 1 in 4 pregnant women carry GBS. Very few babies get sick with GBS disease. (ACOG)

Do I need the test if I am having a planned cesarean?

Yes. Antibiotics for GBS are not needed during a cesarean birth if labor has not started and your water has not broken, but you should still be tested because labor may happen before the cesarean. (ACOG)

Can my baby still get GBS if my test was negative?

Yes. A negative screen does not mean the baby has no risk, because the bacteria are not always detectable by screening the mother. Late-onset disease can also come from contact with other people who have GBS. (MedlinePlus, 2024) (ACOG)

Why is everyone not tested for GBS in the UK?

The UK NSC does not recommend screening because a woman may test positive a few weeks before labour and negative at birth, there is no way of telling which babies will be affected, and many women may be given antibiotics they do not need. (UK NSC)