A group B streptococcus test — also known as a group B strep test — checks to see if you are infected with the group B streptococcus bacteria. It's a common bacterium in the gastrointestinal tract, but can cause serious infections in newborns. If you're pregnant, your health care provider will likely recommend a group B strep test during the third trimester.

During a group B strep test, your health care provider will swab your vagina and rectum and send the samples to a lab for testing. In some cases, you might be given instructions on how to collect the samples yourself. You'll need to repeat the group B strep test each time you're pregnant.

If the group B strep test is negative, no action is needed. If the group B strep test is positive, you'll be given antibiotics during labor to prevent group B strep disease in your baby.

Although group B strep is usually harmless in adults, it can cause complications during pregnancy and serious illness in newborns. The group B strep test is done during pregnancy to identify women who carry the bacterium.

Rarely, however, group B strep can cause health problems during pregnancy, including:

• Urinary tract infection
• Infection of the placenta and amniotic fluid
• Pneumonia
• Bacteria in the blood
• Life-threatening infection in the blood

Group B strep may lead to inflammation and infection of the membrane lining the uterus after delivery, though this is rare. Group B strep also increases the risk of wound infection after a C-section.

The biggest concern is that group B strep can spread to the baby during a vaginal delivery. While only a few babies exposed to group B strep develop an infection, those do develop an infection can develop life-threatening complications. These complications often happen shortly after birth, but sometimes don't develop until days or even months later.

Complications for the baby could include:

• Inflammation of the lungs (pneumonia)
• Inflammation of the membranes and fluid surrounding the brain and spinal cord (meningitis)
• Bacteria in the blood (bacteremia)
• Life-threatening infection in the blood (sepsis)

If you have group B strep, you'll likely be given IV antibiotics during labor — at least four hours before birth. The antibiotics will destroy bacteria in the birth canal and reduce your baby's risk of developing an infection.

Taking antibiotics by mouth or any other route is ineffective. Also, taking antibiotics before labor doesn't help since the bacteria can grow back quickly.

If you previously gave birth to a baby who had a group B strep infection or you had a urinary tract infection caused by group B strep during your current pregnancy, you're at higher risk of spreading group B strep to your baby. As a result, you'll automatically be treated with antibiotics during labor.

Antibiotics aren't necessary if you're having a planned C-section, as long as labor hasn't begun and the amniotic sac — the fluid-filled membrane that surrounds and cushions your baby during pregnancy — is intact. Testing is still important, however, since labor could begin naturally before the scheduled C-section.

A group B strep test is generally done between weeks 36 and 37 of pregnancy. No special preparation is necessary.

Let your health care provider know if you had group B strep in a previous pregnancy and whether you've had a baby with a group B strep infection.

A group B strep test is usually done in a health care provider's office.

During the procedure

You'll lie on your back on an exam table. Your health care provider will use sterile cotton swabs to take samples from your lower vagina and rectum.

Or, you might be given instructions on how to collect the samples yourself.

After the procedure

After the group B strep test, you can return to your usual activities right away.

Test results are typically available within several days.

If you test positive for group B strep, it doesn't mean that you're ill or that your baby will be affected. It simply means you need treatment to prevent an infection in your baby.

Talk with your health care provider about how you'll incorporate your group B strep treatment into your labor plan. It's helpful to remind your health care team of your group B strep status during labor, and to let them know if you're allergic to any medications.