Prenatal Group B Strep (GBS) Screening

A prenatal screening test for group B strep (GBS) is used to detect the presence of this bacteria in the vagina or rectum of pregnant women. About 25% of pregnant women are colonized by GBS. Screening is used to determine the risk that they will pass the bacteria to their newborns during birth, possibly causing a serious infection called early-onset GBS.

The Centers for Disease Control and Prevention (CDC) recommend:

• Screening all pregnant women for the presence of GBS in the vagina and rectum between 35 and 37 weeks of pregnancy unless GBS has already been detected in a urine sample during the pregnancy or the woman has had a previous infant with invasive GBS disease. When GBS is detected during screening, the woman should be treated with intravenous (IV) antibiotics during labor and delivery.
• If a woman has already had GBS detected in a urine sample earlier in her pregnancy or had a previous newborn with GBS, she should automatically receive IV antibiotics during delivery.
• Pregnant women who have not yet been tested for GBS and have preterm labor or whose membranes have ruptured more than 18 hours previously or who have a fever during labor should receive IV antibiotics.

Healthcare practitioners check newborns soon after birth for signs and symptoms of GBS and sepsis, and if present, perform additional testing (e.g., culture of the infant’s blood or spinal fluid) and treating newborns promptly if necessary.

GBS testing may include the following:

• The recommended test for prenatal GBS screening is a culture of the vagina and rectum. For this test, a swab of both sites is placed into a selective enrichment broth, which encourages the growth of any GBS present. If bacteria grow in the broth, additional tests are done to confirm the presence of GBS.
• In certain instances, molecular tests that detect and identify the genetic material of the bacteria directly in vaginal and rectal swabs may be used. Direct testing without broth enrichment is not as sensitive as culture, so these tests are not recommended for routine screening of women in their 35^th to 37^th week of pregnancy and who are not yet in labor. They may be used to screen women who are in labor and have no record of prior testing for GBS and who have no obvious risk factors for the disease. Some factors that can increase risk include premature labor, fever, or prolonged rupture of membranes.
• Urine cultures may be used for screening for significant numbers of GBS.
• Antimicrobial susceptibility testing is not routinely done since GBS is routinely susceptible to ampicillin. However, testing should be performed to determine effective antibiotic therapy for women who are positive for GBS and severely allergic to penicillin and related antibiotics.

Group B strep screening is usually ordered when a woman is 35 to 37 weeks pregnant. A woman can be positive for GBS at one time during her pregnancy and negative at another. That is why it is important to screen a woman late in her pregnancy, closer to the time when she will deliver and could potentially pass the bacteria to her newborn.

Testing should also be performed when a woman goes into labor prior to 35 weeks of pregnancy to determine her GBS status.

A positive test for GBS in a healthy pregnant woman means that the bacteria are currently present in her vagina and/or rectum and she is at risk of passing the bacteria to her infant. She should be given antibiotics intravenously during labor to prevent spreading the bacteria to her baby. This treatment is known as intrapartum prophylaxis.

Pregnant women who have a urine culture positive for significant numbers of GBS any time during their pregnancy should receive appropriate intrapartum prophylaxis to prevent disease in their infants.

A negative result means that the woman is not likely to have GBS in the areas tested. No intrapartum prophylaxis is recommended unless she has other risk factors.

Every pregnant woman should know her GBS status before she delivers and discuss with her healthcare practitioner what antibiotic treatment will be given during her labor if she is positive for GBS.

CDC recommendations represent contributions from and a consensus amongst a large number of national health organizations, including the American College of Obstetricians and Gynecologists, the American Academy of Pediatrics, the American College of Nurse-Midwives, the American Academy of Family Physicians, and the American Society for Microbiology.

Administering antibiotics during labor to those who are colonized with GBS has greatly decreased the incidence of babies born with early-onset GBS infection. A GBS vaccine is not currently available.

No. It is a common bacterium that colonizes both men and women and is not considered an STD.

No. Since about 75% of pregnant women are not colonized by GBS at the time of delivery, it makes the most sense to treat the 25% who are. The numbers of GBS present can vary throughout a woman’s pregnancy, so treatment earlier during a pregnancy does not ensure that GBS is not present at the time of delivery.

Infants can also contract late-onset GBS infection from 7 days to several months after birth. Late-onset disease is not preventable by administering antibiotics during labor since the bacteria are not acquired during pregnancy or during the birth process.

Group A streptococcus (GAS) and group B streptococcus (GBS) are similar types of bacteria, but they cause different types of infections. Group A strep is not usually part of the normal bacterial flora. It is the bacteria that causes “strep throat” that is detected using a rapid strep test or a throat culture. Group A strep may also cause severe skin and wound infections. Group B strep can be part of the normal bacteria found in the throat, vaginal tract, and digestive tract. GBS causes infections in newborns and in adults with weakened immune systems.

Yes. Although GBS usually does not cause illness in healthy adults, it may cause infections of the lining of the uterus (endometritis) in women who have given birth (postpartum) or who have undergone other obstetric or gynecological procedures. GBS also causes wound infections and serious blood and central nervous system infections in adults, especially in those who have diabetes.

GBS can also cause skin infections, pneumonia, septicemia and urinary tract infections (UTI) in the elderly and those with underlying health conditions and/or weakened immune systems, congestive heart failure, vascular disease, liver disease, heart disease, AIDS, or cancer.

General screening of asymptomatic people, as it is discussed in this article, is not performed in these populations. Cultures of the affected areas of the body are done only if signs and symptoms of infection are present.