What is the significance of Group B Streptococcus (GBS) colonization in pregnant women?

Significance of Group B Streptococcus in Pregnant Women

Group B Streptococcus (GBS) colonization in pregnant women is critically significant because colonized mothers are more than 25 times more likely to deliver infants with early-onset GBS disease compared to non-colonized women, making universal screening at 35-37 weeks' gestation and intrapartum antibiotic prophylaxis essential for preventing neonatal morbidity and mortality. 1

Colonization Prevalence and Patterns

• Approximately 10-30% of all pregnant women are colonized with GBS in the vagina or rectum, with the gastrointestinal tract serving as the natural reservoir and source of vaginal colonization. 1, 2

• GBS colonization is typically asymptomatic—most pregnant women have no symptoms associated with genital tract colonization. 1, 2

• Colonization patterns vary and can be transient, chronic, or intermittent, meaning a woman's status can change throughout and between pregnancies. 1, 2

Maternal and Pregnancy Complications

Direct Maternal Risks

• GBS causes urinary tract infections in 2-4% of pregnancies, and GBS bacteriuria at any concentration during pregnancy indicates heavy colonization requiring intrapartum prophylaxis. 1, 2

• Pregnant women can develop amnionitis, endometritis, sepsis, or rarely meningitis caused by GBS during pregnancy or the postpartum period, though maternal fatalities are extremely rare. 1

Pregnancy Outcome Risks

• GBS colonization is associated with preterm birth, preterm premature rupture of membranes, and intrauterine infection. 3

Neonatal Impact: The Primary Concern

Transmission Mechanism

• Vertical transmission occurs primarily after onset of labor or membrane rupture when GBS ascends from the vagina to amniotic fluid, though the organism can also invade through intact membranes. 1

• Fetal aspiration of infected amniotic fluid can lead to stillbirth, neonatal pneumonia, or sepsis. 1

• Infants can also become infected during passage through the birth canal, though most exposed infants become colonized but remain asymptomatic. 1

Disease Risk Quantification

• Without intervention, 1-2% of infants born to colonized mothers develop invasive early-onset GBS disease (occurring in the first week of life). 1, 2

• The case-fatality ratio for early- and late-onset GBS disease is approximately 4% due to advances in neonatal care, down from 50% in the 1970s. 1

• Survivors may experience significant long-term morbidity, particularly those with meningitis, which is more common in late-onset infections. 1, 4

Risk Factors for Neonatal Disease

Beyond maternal colonization, additional risk factors that increase early-onset disease risk include: 1

• Gestational age <37 weeks
• Duration of membrane rupture >18 hours 5
• Intrapartum fever or intra-amniotic infection
• Previous delivery of an infant with invasive GBS disease
• Young maternal age and black race
• Low maternal levels of GBS-specific anticapsular antibody
• History of preterm labor 5
• History of urinary tract infection during current pregnancy 5

Prevention Strategy: Universal Screening

Screening Recommendations

• All pregnant women should undergo vaginal-rectal culture screening at 35-37 weeks' gestation to detect women likely to be colonized at delivery. 1

• Screening earlier in pregnancy is not predictive of neonatal sepsis because colonization status can change. 1

• The optimal screening window (35-37 weeks) balances detection accuracy with proximity to delivery. 1

Proper Specimen Collection

• Swab the lower vagina (vaginal introitus) followed by the rectum (insert swab through anal sphincter) using the same or different swabs. 1

• Cervical cultures are not recommended, and a speculum should not be used for culture collection. 1

• Cultures can be collected by the healthcare provider or the patient with appropriate instruction. 1

Indications for Intrapartum Antibiotic Prophylaxis

Intrapartum prophylaxis is indicated for: 1

• Previous infant with invasive GBS disease (regardless of current screening results)
• Positive GBS screening culture during current pregnancy
• GBS bacteriuria at any concentration during current pregnancy
• Unknown GBS status with any of the following: delivery <37 weeks, membrane rupture ≥18 hours, or intrapartum fever

Intrapartum prophylaxis is NOT indicated for: 1

• Planned cesarean delivery performed before labor onset and membrane rupture (regardless of GBS status)
• Previous pregnancy with positive GBS screening (must rescreen each pregnancy)

Critical Pitfalls to Avoid

• Never treat asymptomatic GBS colonization with antibiotics before the intrapartum period—colonization status can change between treatment and delivery, making prenatal treatment ineffective. 1, 2

• Do not assume GBS status from a previous pregnancy—women colonized in one pregnancy may not be colonized in subsequent pregnancies, requiring screening each time. 1, 6

• However, women with GBS-positive status in their first pregnancy have a 62% cumulative rate of repeated colonization in the second delivery, with cumulative risk of 6.93 compared to those initially negative. 6

• Intrapartum antibiotic prophylaxis is 86-89% effective in preventing early-onset disease, not 100%, so vigilance for neonatal signs of infection remains essential. 1

Emerging Antimicrobial Resistance Concerns

• Significant resistance has been documented to penicillin (29.9%), tetracycline (22.4%), ampicillin (20.9%), and clindamycin (19.4%) in some populations. 5

• Vancomycin remains highly effective, followed by azithromycin, ceftriaxone, and erythromycin. 5

• Multidrug resistance has been reported at 13.4% in certain settings, highlighting the need for antimicrobial susceptibility testing. 5

Future Directions

• Maternal GBS vaccine candidates are in development to address limitations of current intrapartum antibiotic prophylaxis strategies and provide broader protection. 3

• The vertical transmission rate can reach 41.4% among colonized mothers, emphasizing the ongoing need for effective prevention strategies. 5