Patient information: Group B streptococcus and pregnancy

INTRODUCTION

Group B streptococcus (GBS) is a bacterium that can cause serious infections in pregnant women and their babies. This topic review discusses the bacterium, its effects on pregnant women and infants, and interventions that help to prevent GBS infection.

GBS is one of many types of streptococcal bacteria. Another common type of streptococcus is group A streptococcus, which causes strep throat. (See "Patient information: Sore throat in adults".)

GBS is diagnosed by culture, which requires 48 hours to complete. Ideally, the culture should be done at a prenatal visit between 35 and 37 weeks of gestation. Although more rapid tests are available, they are not considered reliable enough to replace cultures.

CAUSE OF GROUP B STREP INFECTION

GBS is commonly found in the gastrointestinal and genital tracts. For healthy adults, the bacterium is not harmful and does not cause problems. In pregnant women and newborn infants, however, GBS infection can cause significant illness.

A person who carries GBS is said to be colonized with the bacteria. Colonization can be temporary, can come and go, or can be chronic. GBS colonization is not harmful and should not be treated, except during labor and delivery (see 'Group B strep prevention and treatment' below.

Between 5 and 40 percent of women (both pregnant and nonpregnant) are colonized with GBS. Although GBS is not a sexually transmitted disease, a person can become colonized during sexual activity. Babies can become colonized from bacteria that are transmitted during labor and delivery and by handling the infant after birth.

GROUP B STREP INFECTION COMPLICATIONS

Pregnancy complications — There is increasing evidence that women who have large amounts of GBS in the vagina and urinary bladder are at a higher risk for developing chorioamnionitis, an infection of the membranes surrounding the fetus. This can lead to premature labor or premature rupture of membranes and preterm delivery.

A urine culture is commonly performed at the first prenatal visit, even for women with no symptoms of urinary tract infection. GBS urinary tract infections, even in pregnant women who have no signs or symptoms (eg, painful, frequent urination) increase the risk of preterm birth. Antibiotics are recommended if a woman's clean catch urine culture shows evidence of infection. Following treatment, a repeat urine culture is recommend to ensure that the infection in the bladder has been eliminated.

Risk factors — Pregnant women are susceptible to infection of the membranes surrounding the fetus (chorioamnionitis), particularly if her water breaks early (several hours or days before birth).

Postpartum complications — A woman can also develop infection related to GBS after delivery. Signs of infection can include fever (often within 12 hours of delivery) and chills, uterine pain and/or a distended abdomen. In rare and severe cases, the infection can spread to the blood (septicemia). Postpartum infections are treated with intravenous antibiotics.

Risk factors — Postpartum infections are more common in women who deliver by cesarean section and in women with preterm labor, premature or prolonged rupture of membranes, frequent cervical examinations, and internal fetal monitoring. Women are susceptible to infection after delivery in areas of injured tissue, such as the lining of the uterus (endometritis) or a cesarean section or episiotomy wound, or in the urinary tract.

Newborn complications — Most newborns who are exposed to GBS from their mother are merely colonized with GBS, and have no signs of infection. Without antibiotic treatment, about 1 to 2 percent of these newborns develop early-onset GBS infection.

Infected newborns can develop pneumonia (lung infection), septicemia (blood infection), or meningitis (infection of the lining of the brain and spinal cord). Premature babies are particularly vulnerable to becoming infected, although most infections with GBS occur in infants born at term (not premature).

Risk factors — Infection in the newborn is more common in the following circumstances:

• Mother has an especially high amount of bacteria in her vagina
• Mother has GBS in her urine
• Baby is born prematurely (less than 37 weeks of gestation)
• Membranes are ruptured for more than 18 hours before birth
• Mother has a fever during labor
• Mother has low levels of antibody against the GBS she carries
• Mother previously had a child with GBS infection
• Premature infants whose mothers have multiple risk factors

Early onset disease — Early-onset GBS infection develops within the first six days of life, although most occur in the first day. Approximately 5 to 10 percent of infected infants die from the disease, with the highest risk of mortality in the most premature newborns.

Late onset disease — Late-onset infection develops between 7 days and 3 months after birth. Half of these infections are the result of GBS transmission from the mother, and the remainder are the result of contact with other GBS carriers in the baby's environment. Late-onset disease now accounts for 50 percent of neonatal GBS infections.

Intravenous antibiotics are the mainstay of treatment of both early and late onset infection. A respirator may be required to assist with breathing. Despite treatment, infants with meningitis are at risk for developmental or neurologic problems, such as seizures, hearing loss, and intellectual disability (mental retardation).

GROUP B STREP PREVENTION AND TREATMENT

Prior to 1992, when programs to prevent GBS transmission began to be used, the overall incidence of newborn GBS infection in the first six days of life (called early-onset infection) was 1 to 3 cases per 1000 live births. Another 0.5 to 1.7 cases per 1000 births occurred in infants from 7 days to 3 months of age (called late-onset infection).

The incidence of early onset infection has decreased substantially, to 0.34 infants per 1000 births, since prevention and treatment guidelines were implemented.

GBS culture — Expert groups recommend that all pregnant women have a GBS culture of the vagina and rectum at 35 to 37 weeks of gestation. This is not necessary if the woman had GBS in a urine culture earlier in the current pregnancy or for women who previously had an infant with GBS infection; these women should always be given antibiotics during labor and delivery.

Cultures taken at 35 to 37 weeks of gestation can predict if GBS colonization is likely during labor and delivery. Cultures done more than five weeks before delivery are less reliable for predicting GBS colonization at delivery.

Antibiotic treatment — The American Academy of Pediatrics, American College of Obstetricians and Gynecologists, and Centers for Disease Control and Prevention all recommend antibiotics during labor for all GBS colonized pregnant women (see 'Newborn complications' above.

If a woman has a positive GBS culture and is she allergic to penicillin (eg, there is a risk of a life-threatening reaction), she should be sure that her clinician is aware of this allergy. An additional test may be needed to determine which antibiotic should be used during labor.

Benefits — Antibiotics given in labor reduce the chance of early-onset neonatal infection 30-fold. If antibiotics are not given to a GBS-colonized woman at term who has no other risk factors (eg, fever, prolonged membrane rupture), there is a 1 in 200 chance of neonatal GBS infection; this risk drops to 1 in 4000 if antibiotics are given. Antibiotics given during labor do not reduce the risk of late-onset infant infection.

Risks — Although relatively safe, antibiotics can potentially have side effects. For penicillin, there is a 1 in 10 chance of a minor allergic reaction such as a rash, a 1 in 10,000 chance of a severe, life-threatening reaction (called anaphylaxis), and a 1 in 100,000 risk of a fatal reaction. For the overwhelming majority of women, the benefits of antibiotics to prevent neonatal GBS outweigh this risk.

SUMMARY

• Group B streptococcus (GBS) is a bacterium that is commonly found in the gastrointestinal and genital tracts. For healthy adults, the bacterium is not harmful and does not cause problems. However, for pregnant women and newborn infants, GBS can cause infection and significant illness.
• A person who carries GBS is said to be colonized. Between 5 and 40 percent of women (both pregnant and nonpregnant) are colonized with GBS. GBS colonization is not harmful and cannot be eliminated with antibiotic treatment. Giving antibiotics at the time of delivery, however, can prevent infection in the baby (see 'Cause of Group B strep infection' above.
• A pregnant woman who develops GBS infection is at increased risk of premature labor or premature rupture of membranes and preterm delivery. GBS urinary tract infections, even in pregnant women who have no signs or symptoms (eg, painful, frequent urination) increase the risk of preterm birth (see 'Pregnancy complications' above.
• An infant can become colonized from bacteria that are transmitted during labor and delivery. In 2004, approximately 0.34 per 1000 infants became infected with GBS. Infected babies can develop pneumonia (lung infection), septicemia (blood infection), or meningitis (infection of the lining of the brain and spinal cord). Premature babies are particularly vulnerable to becoming infected, but the infection also occurs in term babies (see 'Newborn complications' above.
• Expert groups recommend that all pregnant women have a GBS culture of the lower vagina and rectum at 35 to 37 weeks of gestation. This is not necessary if the woman previously had GBS in a urine culture during the current pregnancy or if the woman previously gave birth to an infant with GBS infection (these women should always receive antibiotics during labor and delivery) (see 'Group B strep prevention and treatment' above.
• Experts recommend antibiotics during labor for pregnant women who are colonized with GBS and those whose infants are at high risk of early-onset neonatal GBS infection. Antibiotics given in labor reduce the risk of early-onset neonatal infection 30-fold.

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem. Because no two people are exactly alike and recommendations can vary from one person to another, it is important to seek guidance from a provider who is familiar with your individual situation.

This discussion will be updated as needed every four months on our web site (www.uptodate.com/patients). Additional topics as well as selected discussions written for healthcare professionals are also available for those who would like more detailed information.

Some of the most pertinent include:

Patient Level Information:
Patient information: A guide to pregnancy
Patient information: Sore throat in adults

Professional Level Information:
Chemoprophylaxis for the prevention of neonatal group B streptococcal disease
Group B streptococcal infection in neonates and young infants
Group B streptococcal infection in pregnant women
Group B streptococcal infections in nonpregnant adults
Group B streptococcus: Virulence factors and pathogenic mechanisms
Vaccines for the prevention of group B streptococcal disease
Patient information: A guide to pregnancy

A number of web sites have information about medical problems and treatments, although it can be difficult to know which sites are reputable. Information provided by the National Institutes of Health, national medical societies and some other well-established organizations are often reliable sources of information, although the frequency with which they are updated is variable.

• National Library of Medicine

      (www.nlm.nih.gov/medlineplus/healthtopics.html)

• The Mayo Clinic

      www.mayoclinic.com/health/group-b-strep/PR00079

• Group B Strep Association

     (www.groupbstrep.org)

• The Center for Disease Control and Prevention (information in English and Spanish)

      www.cdc.gov/groupbstrep/

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