Group B Strep Test During Pregnancy: What to Expect

    Published 29 July 2020
    Fact Checked
    Reviewed by Olga Adereyko, MD, Primary Care Physician, General Practitioner, Medical Consultant
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    During pregnancy, common bacteria like group B strep (GBS) can cause health problems for the mother and baby. To identify group B strep, many pregnant women take the group B strep test during the third trimester of their pregnancy. This helps prevent health complications that might arise during or after the birth.

    What is a group B strep test?

    The strep B test is used to determine whether pregnant women carry the common bacteria known as streptococcal bacteria. This bacteria, found in approximately two in five people, is usually harmless. For pregnant women, however, strep B can lead to complications. That’s why health care providers usually recommend that pregnant women take group B strep tests as a precaution.

    Group B strep is typically found in the lower genital tract or intestines. During a group B strep test procedure, your health care provider will take samples from your rectum and vagina using a clean cotton swab. If your samples are negative for group B strep, you probably don’t need to take any precautions. If your samples are positive, your health care provider may prescribe medications during labor to prevent future complications for you and your baby.

    Why the group B strep test is done

    If you carry the streptococcal bacteria during pregnancy, it doesn’t necessarily mean that you’re sick. However, it can pose a risk to you, the pregnancy, and the baby. This bacteria can cause infections in the baby’s blood, lungs, and brain. Taking the group B strep test and any treatments prescribed by your health care provider can reduce the risk of dangerous infections.

    Group B streptococcus risks during pregnancy

    Group B streptococcus can cause inflammation and infection in the mother and baby. This condition is often referred to as GBS disease. For the mother, this can lead to several health problems throughout the pregnancy, including sepsis, bacteremia, pneumonia, and urinary tract infection. If you have a C-section, the presence of group B strep can lead to an infection in the wound after delivery. It can also increase the risk for infection of the amniotic fluid and placenta (chorioamnionitis) and infection of the uterine membrane lining (endometritis). 

    On the day you go into labor, certain factors can increase your risk of GBS. If you go into labor or your water breaks before the 37th week of pregnancy, you may not have been tested or received the results of your group B strep test yet. Long deliveries of more than 18 hours with premature rupture of membranes and fevers higher than 100.4 degrees Fahrenheit can also increase your risk. If you had GBS complications with previous pregnancies, you are more likely to pass the bacteria during your upcoming delivery. 

    Strep B poses health risks to pregnant women, but this bacteria places their babies at greater risk. Babies can be exposed to strep B during vaginal delivery when they come into contact with the fluids containing the bacteria. For most babies, this exposure doesn’t lead to infection. Group B strep only affects about 1 in 1,750 babies. For babies who are exposed, they may experience complications such as pneumonia, sepsis, bacteremia, and meningitis. These health issues can show up just after the baby is born or months afterward. 

    Babies who are exposed to strep B usually develop one of the two types of GBS disease. Early-onset GBS develops within the first week of the baby’s life, and the late-onset variety develops within the first three months. Thanks to effective preventive measures, both types of GBS are now rare. Approximately 900 babies born in the United States develop the early-onset disease per year, and approximately 1,200 babies develop the late-onset disease. Undergoing the group B strep test at the right time in your pregnancy can help protect you and your baby before, during, and after labor.

    When to undergo the group B strep test

    Health care providers usually perform group B strep tests during the third trimester, specifically between the 36th and 37th weeks of pregnancy. By testing in the weeks leading up to the birth, your health care providers ensure they have up-to-date information when you go into labor. 

    The strep B test is a relatively simple procedure, and it doesn’t require any preparation. If this isn’t your first pregnancy, make sure to inform your health care provider about your previous group B strep results. Let them know whether you tested positive or negative for the bacteria and whether your baby was affected by it.

    Group B strep test procedure

    Group B strep tests are quick, easy, and painless. The procedure is usually performed in your health care provider’s office, although they may give you the option to collect the samples yourself at home. 

    For your health care provider to conduct the procedure, you must first lie flat on the exam table. Using sterile swabs, your health care provider will collect samples from the areas where streptococcal bacteria are most commonly found: the rectum and vagina. Once the health care provider has collected enough cells and secretions, you can go back to your normal routine. You’ll receive the results of your strep B test a few days later.

    What if you test positive for group B strep?

    There are health risks associated with strep B, but a positive test result only indicates the potential for infection. With proper treatment, you can minimize that risk, leading to a healthy outcome for you and your baby. 

    If you do test positive for group B strep, your health care provider will probably prescribe intravenous antibiotics during labor. There is no vaccine or pre-birth treatment you can take to protect against this bacteria. Because it is fast-growing and present in the birth canal, prescribing antibiotics before labor or by mouth isn’t very effective. 

    To protect the baby from strep B exposure during labor, health care providers usually start pregnant women on a round of intravenous antibiotics about four hours before birth. This helps destroy any bacteria in the birth canal by the time the baby is born. When pregnant women test positive for group B strep bacteria, receiving antibiotics during labor can lower the risk of their baby developing GBS disease to 1 in 4,000. 

    During labor, babies can be exposed to group B strep when the amniotic sac breaks. If you plan to deliver via C-section, you may not need to take antibiotics, even if you’ve tested positive for strep B. You should take the strep B test even if you’ve scheduled a C-section just in case you go into labor early.

    The takeaway

    Effective treatments have made GBS disease rare in newborns, regardless of whether the mother carries the group B strep bacteria. To safeguard the health of your baby, make sure to take the group B strep test near the end of your pregnancy and accept delivery-day antibiotics if necessary. These measures are the best ways to protect you and your baby against potential health risks.

    History of updates

    Current version (29 July 2020)

    Reviewed by Olga Adereyko, MD, Primary Care Physician, General Practitioner, Medical Consultant

    Published (29 July 2020)

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