Group B Streptococcus Infection and Preterm Birth

Mechanism and Risk Factors

The Centers for Disease Control and Prevention recommends that Group B Streptococcus (GBS) infection is directly associated with preterm birth and is recognized as a significant risk factor for preterm delivery in pregnant women, with GBS ascending from the vagina to the amniotic fluid after onset of labor or rupture of membranes 1, 2
The American College of Obstetricians and Gynecologists notes that the gastrointestinal tract serves as the primary reservoir for GBS, which then colonizes the vaginal tract in approximately 10-30% of pregnant women 1

The Centers for Disease Control and Prevention states that GBS bacteriuria during pregnancy is a marker for heavy genital tract colonization and has been associated with increased risk for preterm birth 2
The American College of Obstetricians and Gynecologists recommends that when labor or rupture of membranes occurs before 37 weeks' gestation with unknown GBS status, perform GBS screening immediately and initiate intrapartum antibiotic prophylaxis pending culture results 2
Intrapartum antibiotic prophylaxis administered for ≥4 hours before delivery to women delivering at <37 weeks' gestation is 78% effective (95% CI: 44%-91%) in preventing early-onset GBS disease, according to the Centers for Disease Control and Prevention 2

Group B Streptococcus (GBS) is the leading infectious organism associated with preterm birth and represents the most common cause of early-onset neonatal sepsis in the United States, with GBS colonizing 10-30% of pregnant women in the vaginal or rectal tract 3
Mortality is substantially higher among preterm infants, with case-fatality rates of approximately 20% and as high as 30% among those ≤33 weeks' gestation, compared with 2-3% among full-term infants 3

High-risk indicators for GBS-associated preterm birth include gestational age <37 weeks at onset of labor or membrane rupture, membrane rupture >12 hours before delivery, intrapartum fever, and previous delivery of an infant with invasive GBS disease 3
Women with GBS colonization are more than 25 times more likely to deliver infants with early-onset GBS disease compared to those with negative cultures 3

Heavy GBS colonization is associated with a higher risk for early-onset disease 3