Screening for Group B Streptococcus in Pregnancy

Written by the Healthline Editorial Team | Published on March 15, 2012
Medically Reviewed by Dominic Adam Marchiano, MD

What Do I Have?

Group B streptococcus (GBS) is a bacterium that can cause serious infections in newborns and pregnant women. This bacterium is often found in the mouth and throat, lower-intestinal tract, and vagina of healthy women. It is considered part of the normal bacteria found in healthy individuals. GBS is not a sexually transmitted disease (STD).

GBS ( Streptococcus agalactia ) should not be confused with Group A streptococcus. Group A streptococcus ( Streptococcus pyogenes ), is the bacterium that causes "strep throat" and, other serious infections.

How Can a Type of Bacteria Normally Present in the Body Cause Illness?

When discussing the relationship between bacteria and the human body, it is important to keep in mind that we normally "host" billions of bacteria-collectively referred to as the normal bacterial flora. Such bacteria often provide beneficial effects for the human (or host). We also have natural "host defenses" against these bacteria, which keep the bacterial population from growing too large and limit the presence of bacteria to external organs (skin), the alimentary canal (from the mouth to the rectum), and the lower part of the genital tract (in women, the vagina and cervix). Such host defenses include intact skin and the lining of the alimentary canal, immune cells (white blood cells), and antibodies (produced by the immune system to combat infectious organisms) secreted into these areas.

When bacteria live in harmony with a certain area of the human body, the bacteria are said to "colonize" that region of the body. But, if there is a breakdown in these host defenses-such as a surgical incision causing a break in the skin-bacteria that normally reside on the outer portions of the body may enter the sterile regions, multiply, and damage the host.

Pregnancy increases the risk of GBS infection to both mother and baby. When pregnant, your immune system does not function quite as well as a non-pregnant woman's immune system. Moreover, labor and delivery are the most susceptible times for both mother and baby to acquire bacterial infections.

Pregnant women with vaginal GBS are referred to as "GBS colonized" or "GBS carriers." In certain circumstances, the GBS bacteria in these women invade more than just the intestinal tract or vagina. When the number of GBS bacteria present in areas of the body other than the vagina or intestine is great enough to cause physical illness, the individual has a "GBS infection."

What Type of Infections Can GBS Cause?

Most people who have colonized GBS bacteria in their bodies do not suffer consequences. However, GBS can cause a variety of infections in adults and newborn babies.

  • Infections in adults-GBS can cause infections of the bladder, uterus, and surgical incisions. Occasionally, it can cause an infection in the bloodstream (sepsis).
  • Infections in newborns-GBS is one of the most common types of bacteria to cause serious infections in newborns, though, not as often as in the past. Before the widespread use of prevention methods, GBS disease affected one or two out of every 1,000 babies born in the U.S. Fortunately, the rate of GBS disease in newborns has declined over the last few years due to a nationwide prevention effort. Nonetheless, in newborn babies, Group B Strep can cause serious infections, including sepsis (infection of the blood stream); meningitis (infection of the lining of the spinal cord and brain); pneumonia (infection of the lung); and skin infections.

An infection that occurs during the first seven days of life is known as an early-onset GBS infection. These infections usually result from infection during the birth process. Infections after the first week of life are referred to as a late-onset GBS infection. About one-half of these infections are caused by GBS bacteria acquired during the birth process, while the other half are caused by GBS acquired after birth (usually from contact with colonized individuals other than the mother). Late-onset GBS infections are less common than early-onset infections.

Talking About Risk.

Some infants of mothers colonized with GBS are more likely than others to develop a GBS infection. Your baby is at high risk for developing GBS, if you:

  • deliver a premature infant (prior to 37 weeks gestation);
  • had a long labor, particularly if the membrane (bag of waters) was ruptured for more than 18 hours;
  • had a GBS bladder infection during pregnancy;
  • have previously delivered an infant who developed GBS disease; and
  • had a fever during labor.

Further, a baby delivered to a mother who carries GBS and has at least one of these risk factors can have up to a one in 20 chance of having newborn GBS disease-compared to only a one in 200 chance for a baby whose mother carries GBS, but has none of these risk factors.

How Serious Is a GBS in Newborns?

The mortality rate for infants who have a serious GBS infection during the first week of life is one in 10. Babies with GBS infections often require close monitoring and special treatment in an Intensive Care Nursery. Some infants who survive GBS infections suffer permanent disabilities. Among babies with GBS meningitis, approximately 20% develop problems such as seizures, developmental delays, or mental retardation. Babies who survive GBS infections of the bloodstream (sepsis) or lungs (pneumonia) are less likely to suffer long-term problems than babies who have had GBS meningitis.

How Does a Pregnant Woman Know Whether She Is Colonized with GBS?

The easiest way for your doctor to determine whether you have GBS in the birth canal is to obtain a specimen from the lower portion of your vagina and the outside of the rectum. This specimen will be cultured (grown in a laboratory) and analyzed. Results are generally available in two to three days. A positive GBS culture only indicates that you are colonized with GBS; it does not necessarily mean you or your baby will develop a serious infection.

When Should the Culture Be Taken?

Having GBS in your body does not mean that you will have a positive culture of the lower vagina and rectum. Studies show that of the women who had GBS cultures every few weeks throughout their pregnancy, only one-half of the women who had GBS in their birth canal at the time of delivery had previously had some negative cultures. So, some women appear to have GBS bacteria in the birth canal only intermittently.

Since the presence of GBS in the birth canal during labor poses significant risk of infant infection, a culture performed within a few weeks of delivery is of greatest value in predicting whether or not GBS will be present at the time of delivery. Since the majority of women deliver between the 37th and 41st weeks of gestation (as measured from the first day of the woman's last menstrual period), most doctors perform GBS cultures between a woman's 35th and 36th week of pregnancy.

What Other Tests Can Detect GBS?

There are tests currently available which can detect the presence of GBS within several hours; these are commonly referred to as rapid tests. In general, rapid tests are quite accurate in determining that GBS is present. Unfortunately, a negative rapid test result does not exclude the possibility that you are colonized with GBS-especially if you have only a small amount of GBS bacteria in your birth canal. Overall, these tests are not as accurate as cultures in detecting GBS.

What If My Doctor or Midwife Doesn't Perform a GBS Culture on Me?  

Most major agencies overseeing the care of pregnant women, including the CDC and the of Obstetricians and Gynecologists, feel that a culture should be performed on all pregnant women presenting for care during the window of opportunity. Screening by culture is thought to be superior to prescribing antibiotics only for women whose infants are at risk for GBS disease.

When, for whatever reason, a GBS culture is not performed prior to labor, a woman may be screened by a rapid GBS test or she may be treated based on risk factors. It's a good idea to discuss GBS cultures with your doctor or midwife during the early part of your pregnancy.

How Can GBS Infections Be Treated?

Fortunately, GBS is sensitive to readily available antibiotic drugs such as penicillin (PenVK) and ampicillin (Principen). Penicillin-allergic patients may be treated with clindamycin, erythromycin, or vancomycin. Therefore, the most important aspect of treatment is early recognition of serious infection and initiation of standard antibiotic drugs.

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