GBS overview

Group B streptococcus, also called GBS, is a common bacterium in your intestines or lower genital tract. It’s usually harmless in adults. However, it can cause serious illness in newborns.

GBS can be found in the vagina or rectum of about 25 percent of healthy pregnant women. Those infected may not always show symptoms. So, if you’re pregnant it’s recommended you get screened for GBS in your third trimester.

If you have the bacteria in your system when you go into labor, there’s a risk of the bacteria being transmitted to your baby during delivery.

The GBS screening process

All women who are expected to deliver prematurely are given antibiotics during labor. These are women expected to deliver before the 37th week of pregnancy. All other women will have a routine GBS culture. This will happen between their 35th and 37th weeks of pregnancy.

For the culture, your doctor will take a swab sample from the lower portion of your vagina and the outside of your rectum. The sample will be sent to a lab for analysis. If your culture is positive, you will be given antibiotics intravenously during labor. Antibiotics will typically be started at least four hours before delivery.

If I am given antibiotics, will my baby need to take antibiotics?

If you are given antibiotics during labor, it will lower the chance of your baby contracting GBS. The antibiotics used are penicillin (PenVK) or ampicillin (Principen).

For women who have had an allergic reaction to penicillin, cefazolin may be used. In cases of a more severe allergic reaction, clindamycin or vancomycin will be used.

If you receive antibiotics at least four hours before delivery, your baby probably won’t need antibiotics. If you were given antibiotics less than four hours before delivery, or if your doctor suspects an infection, your baby may need to stay in the hospital for observation and blood tests.

The number of white blood cells in your baby’s blood will be measured. Also, a blood culture and a chest X-ray may be involved, and your baby may receive antibiotics as a preventive measure.

Why shouldn’t I be treated for GBS before I go into labor?

There is a span of about four to five weeks when most women go into labor. This is usually between the 37th and 42nd weeks of pregnancy. This means antibiotics could be taken for four to five week before labor.

Taking an antibiotic drug for several weeks could cause serious side effects. Instead, antibiotics are only prescribed to treat GBS before labor if a woman has a bladder infection caused by GBS.

Why don’t all women receive antibiotics to prevent GBS infection?

Any antibiotic can cause side effects or reactions. Generally, side effects from antibiotics are mild and don’t cause harm. However some people can have a severe or even life-threatening reaction to them.

Another concern is that giving antibiotics could result in an increased chance for serious infections in newborn babies from other bacteria.

Your body has both “good” and “bad” bacteria in it. Antibiotics target the bad bacteria. However, sometimes antibiotics only wipe out the good bacteria. This is called selection pressure. When the good bacteria are killed off, they are naturally selected out. This allows the bad bacteria to grow.

Treating infections can be difficult. Some people with selection pressure bacteria need powerful antibiotics since the bacteria are resistant to standard antibiotics.

Generally, it is recommended that only women who have GBS, or have a significant risk factor for their infant developing a GBS infection, should get antibiotics during labor.

However, all women who have GBS in their urine during pregnancy, or who have already had a child with a GBS infection, are given antibiotics during labor. They may not even be tested.

Will antibiotics given during labor prevent all cases of newborn GBS disease?

Giving antibiotics to women in labor who are at risk of having an infant with GBS will reduce the chances of infection in their newborn. However, it doesn’t completely eliminate the possibility. Antibiotics are not 100 percent effective in preventing newborn GBS infections.

If you have GBS and receive antibiotics, you have a 1 in 4,000 chance of passing it on to your baby, according to the American Pregnancy Association. If you don’t receive antibiotics, there is a 1 in 200 chance.

Babies delivered to women with a high fever due to infection in the womb are particularly at risk for developing GBS. This is true even when antibiotics are used during labor.

There is also a chance that a woman with GBS will have had a negative culture. This is sometimes true even if the culture was done several days or weeks before delivery.

In other cases, some women have short labors. They deliver soon after arriving at the hospital or birth center. This may not allow time to administer the antibiotics.


GBS is a common bacterium in pregnant women. It can be hard to detect because you may not show any symptoms. This infection isn’t hard to treat in adults, but it can be serious for newborns. So, it’s important to get screened in your third trimester. Although antibiotics may have some side effects, they are effective in treating GBS and preventing the bacteria from being passed on to your newborn.