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Prenatal Group B Strep Screening

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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

At a Glance

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?


The Test Sample

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.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Quest Diagnostics. Streptococcus Group B Culture. Available online at through Accessed June 2013.

Centers for Disease Control and Prevention. Fast Facts and Statistics on GBS. Available online at through Accessed June 2013.

Centers for Disease Control and Prevention. Symptoms, Diagnosis, and Treatment of GBS in Newborns. Available online at through Accessed June 2013.

Centers for Disease Control and Prevention. GBS Prevention in Newborns. Available online at through Accessed June 2013.

Bacterial Infections and Pregnancy: Group B Streptococcus. Medscape. Available online at through Accessed June 2013.

Sources Used in Previous Reviews

Woods, C. and Levy, C. (Updated 2010 May 17). Streptococcus Group B Infections. eMedicine [On-line information] Available online at through Accessed October 2010.

Comer, Yun, H. and Hamza, H. (Updated 2010 February 11). Bacterial Infections and Pregnancy. eMedicine [On-line information] Available online at through Accessed October 2010.

Mayo Clinic Staff (2008 December 5). Group B strep [On-line information] Available online at through Accessed October 2010.

Lee, K. and Zieve, D. (Updated 2009 December 10). Group B streptococcal septicemia of the newborn. MedlinePlus Medical Encyclopedia [On-line information] Available online at Accessed October 2010.

(Reviewed 2008 April 20). Group B Strep Prevention (GBS, baby strep, Group B streptococcal bacteria) General Public, Frequently Asked Questions. Centers for Disease Control and Prevention [On-line information]. Available online at through Accessed October 2010.

Bush, L. et. al. (Revised 2009 December). Streptococcal and Enterococcal Infections. Merck Manual for Healthcare Professionals [On-line information] Available online at b strep&alt=sh through Accessed October 2010.

(2010 March). Group B Strep Infection. March of Dimes Fact Sheet [On-line information] Available online at through Accessed October 2010.

Forbes, B. et. al. (© 2007). Bailey & Scott's Diagnostic Microbiology, 12th Edition: Mosby Elsevier Press, St. Louis, MO. Pp 265-279.

(November 19, 2010) Centers for Disease Control and Prevention, Morbidity and Mortality Weekly Report, Prevention of Perinatal Group B Streptococcal Disease, 59(RR10);1-32. Available at through Accessed November 2010.

Centers for Disease Control and Prevention. Group B Strep (GBS). Available online at through Accessed November 2010.