Patient Test Information

Prenatal Group B Strep Screening

Also known as:

Group B Beta-hemolytic Streptococcus; Group B Streptococcus; Group B Strep; GBS; Strep Group B; Beta-hemolytic Strep Culture

Formal name:

Prenatal Group B Streptococcus Screening

Related tests:

Urine Culture; Gram Stain; Susceptibility Testing

Why Get Tested?

To screen a pregnant woman for the presence of group B streptococcus (GBS) to determine the risk that she will pass the bacteria on to her newborn, possibly resulting in a serious infection

When to Get Tested?

When a pregnant woman is between 35 and 37 weeks of gestation or has preterm labor or preterm premature rupture of membranes

Sample Required?

A culture swab obtained from the vagina and rectum of a pregnant woman; a urine sample collected anytime during pregnancy may be used to detect significant numbers of GBS.

Test Preparation Needed?

None

How is it used?

A prenatal screening test for group B streptococcus (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.

The Centers for Disease Control and Prevention (CDC) released updated recommendations in 2010 for the prevention of perinatal group B streptococcal disease. The guidelines outline procedures for the universal screening of pregnant women at 35 to 37 weeks gestation to detect GBS colonization. The report also provides details on the recommendation for antibiotic treatment during labor for those who are colonized in order to prevent early-onset GBS disease in newborns. The guidelines contain provisions for:

  • Screening all pregnant women for vaginal and rectal GBS colonization at 35 to 37 weeks gestation unless GBS has already been isolated from a urine sample during the pregnancy or the woman has had a previous infant with invasive GBS disease; in these circumstances and when GBS is positive, the woman should be treated with antibiotics during delivery.
  • Testing of those who have preterm labor and/or premature rupture of amniotic membranes at less than 37 weeks gestation; those whose membranes have ruptured more than 18 hours previously are at an increased risk for GBS.
  • Identifying significant quantities of GBS in the urine of asymptomatic pregnant women.
  • Observing newborns for signs of sepsis and testing (culture of the infant's blood or spinal fluid) and treating them promptly.

Guidelines from the CDC also expanded the methods used by laboratories to process samples for GBS screening in order to improve detection. Recommendations 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. After the bacteria have been grown and isolated in culture, additional tests will be performed to confirm the presence of GBS. The updated guidelines offer alternative identification tests to enhance detection of the bacteria and to decrease the turnaround time to obtain the final results. In certain instances, molecular tests that detect and identify the genetic material of the bacteria may be employed.
  • Current molecular assays performed directly on the vaginal/rectal swab without an enrichment step are not as sensitive as culture. Therefore, these molecular tests are not recommended for routine screening of women in their 35-37th week of pregnancy and who are not yet in labor. Molecular testing of the vaginal/rectal sample is permitted to 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 colony counts of GBS.
  • Antimicrobial susceptibility testing is not routinely ordered when GBS is isolated; however, testing should be ordered to evaluate alternative treatments if a woman with GBS colonization is severely allergic to penicillin.

When is it ordered?

Prenatal GBS screening is usually ordered on pregnant women at 35 to 37 weeks gestation per CDC recommendations. 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 if a woman goes into labor prior to 35 weeks gestation in order to determine her GBS status.

Antimicrobial susceptibility testing should be ordered whenever a woman is severely allergic to the standard GBS treatment, penicillin or ampicillin, in order to select an effective alternative therapy.

What does the test result mean?

A positive test for GBS in a healthy pregnant woman means that she is currently colonized with GBS and should be given antimicrobial agents intravenously during labor to prevent the transmission of the bacteria to the 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 be colonized in the areas tested. No intrapartum prophylaxis is recommended.

Is there anything else I should know?

Every pregnant woman should know her GBS status before she delivers and discuss with her doctor what prophylaxis 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.

What is being tested?

Streptococcus agalactiae, also known as group B streptococcus (GBS), is a common bacterium that colonizes the gastrointestinal tract and genital tract. It rarely causes symptoms or problems in healthy adults but can cause infections and serious illness in newborns. Group B strep screening identifies the presence of the bacteria in the vaginal/rectal area of a pregnant woman.

GBS can be passed from mother to child before or during delivery, causing early-onset GBS disease that appears within hours to days of birth. Symptoms in newborns include fever, difficulty with feeding and breathing, irritability or lethargy, and a blue tint to their skin. GBS can cause serious infections such as pneumonia, sepsis, and meningitis. According to the Centers for Disease Control and Prevention, it is the most common cause of life-threatening infections in newborns.

Approximately 25% of pregnant women carry group B strep in their rectum or vagina. However, the number of infants with GBS disease has decreased significantly in recent years because of a concerted effort by health care providers to screen pregnant women for GBS late in their pregnancy and, when they are positive for GBS, to treat them with intravenous antibiotics (usually penicillin or ampicillin) during labor. This prevents or greatly decreases the risk of passing the bacteria to the newborn. Nevertheless, GBS disease remains the primary cause of early-onset sepsis, a serious and life-threatening infection in newborns.

Currently there is no vaccine available to prevent GBS, and treating all pregnant women with antibiotics is not practical. Screening for GBS and appropriate treatment continues to be the best means for preventing GBS disease in newborns.

How is the sample collected for testing?

For screening pregnant women, a swab is typically obtained from the vagina and rectum. Urine collected during pregnancy may be cultured for significant numbers of GBS.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

  1. Is it possible to prevent a group B strep (GBS) infection?

    Administering antimicrobial agents 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.

  2. Is GBS considered a sexually transmitted disease (STD)?

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

  3. Should every woman be treated with antibiotics for GBS during delivery or during pregnancy?

    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. Since GBS colonization comes and goes, treatment during a pregnancy would not ensure that GBS is not present at the time of delivery.

  4. Can babies get GBS infections more than 7 days after they are born?

    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.

  5. What is the difference between group A and group B streptococcus?

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

  6. Does GBS cause serious infections in adults?

    GBS 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 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.