Associated Women's Health Specialists  

Group B Streptococcus in Pregnancy

Group B streptococcus (GBS) is a type of bacteria that can be found in 10–30% of pregnant women. A woman with GBS can pass it to her baby during delivery. Most babies who get GBS from their mothers do not have any problems. A few, however, will become sick. This can cause major health problems or even threaten their lives. This pamphlet explains:
  • How GBS may affect a newborn
  • How to test for GBS
  • How it can be treated
GBS is fairly common in pregnant women. Treatment during labor and delivery may help prevent infection in your baby.
What Is GBS?

GBS is one of the many bacteria that usually do not cause serious illness. It may be found in the digestive, urinary, and reproductive tracts of men and women. In women, it is most often found in the vagina and rectum. GBS is not a sexually transmitted disease.

A person who has the bacteria but shows no symptoms is said to be colonized. If the bacteria grow and cause symptoms, infection has occurred. Although the names are similar, GBS is different from group A streptococcus, the bacteria that causes "strep throat."

In most cases, women who are colonized with GBS are not in any health danger and will not need to be treated. However, if a woman is pregnant, she can pass GBS to her baby. For this reason, a woman will be tested and, if GBS is present, treated during labor.

Effects on the Baby

If the bacteria is passed from a woman to her baby, the baby may develop GBS infection. This happens to only 1 or 2 of every 100 babies whose mothers have GBS. Babies who do become infected may have early or late infections.

Early infections develop right after delivery when the baby passes through the birth canal colonized with GBS. Early infections occur within the first 7 days after birth. Most occur within the first 6 hours after birth.

Late infections occur after the first 7 days of life. About one half of late infections are passed from the mother to the baby during birth. The other half result from other sources of infection, such as contact with other people who are GBS carriers.

Both early and late infection can be serious. These infections can cause inflammation of the baby's blood, lungs, brain, or spinal cord. Both early and late GBS infections lead to death in about 5% of infected babies.

Testing for GBS

A culture is the most accurate way to test for GBS. This is a simple procedure and should not be painful. With cultures, a swab is placed in the woman's vagina and rectum to obtain a sample. The samples then are sent to a lab where they are grown in a special substance. It may take up to 2 days to get the results.

The results of cultures are most useful between 35 and 37 weeks of pregnancy. If the test results are positive, showing that GBS is present, you will receive antibiotics during labor to help prevent GBS from being passed to your baby.


To reduce the risk of GBS infection in newborns, all women who test positive for GBS must be treated with antibiotics during labor. Babies of women who are carriers of GBS and do not get treatment have more than 20 times the risk of getting infected than those who do receive treatment.

Antibiotics help get rid of some of the bacteria that can harm the baby during birth. The antibiotics work only if taken during labor. The bacteria grow so fast that if treatment is given earlier, the GBS may grow back before labor. If you had a previous baby with GBS infection or you had a urinary tract infection caused by GBS during pregnancy, you do not need to be tested. You will need to get antibiotics during labor. Antibiotics are given through a vein.

Penicillin is the antibiotic that is most often given to prevent GBS in newborns. Another antibiotic may be given if you are allergic to penicillin. Although most of the time there are no side effects to penicillin, 1 out of every 10 women may have a mild rash in response to this treatment. Many times this happens within 1 to 2 weeks after treatment. Rarely, a woman may have a severe allergic reaction and require emergency treatment. Inform your doctor if you develop a rash after treatment.

In women who have planned a cesarean birth, it is not necessary for them to be given antibiotics during delivery, whether or not they are GBS carriers. However, these women should still be tested for GBS because preterm labor may occur before the planned cesarean birth.

Talk to your doctor about your GBS status. Pregnant women who do not know if they are GBS positive or have not yet been tested when labor starts should be given antibiotics if:

  • They have preterm labor
  • They have a fever during labor
  • Their water breaks 18 hours or more before delivery occurs. This means the fluid-filled amniotic sac that has cushioned the baby in the uterus throughout pregnancy has ruptured.

Your doctor can advise you on the best approach.


GBS is fairly common in pregnant women. Yet, very few babies actually become sick from GBS infection. Treatment during labor and delivery may help prevent infection in your baby.


Amniotic Sac: Fluid-filled sac in the mother's uterus in which the fetus develops.

Carriers: Persons who are infected with the organism of a disease without showing symptoms and who can transmit the disease to another person.

Cesarean Birth: Delivery of a baby through an incision made in the mother's abdomen and uterus.

Colonized: Having a bacteria in your body that could cause illness, but having no symptoms of the disease.

Inflammation: Pain, swelling, redness, and irritation of tissues in the body.

Sexually Transmitted Disease: A disease that is spread by sexual contact, including chlamydial infection, gonorrhea, genital warts, herpes, syphilis, and infection with human immunodeficiency virus (HIV, the cause of acquired immunodeficiency syndrome [AIDS]).