Can GBS Infection Cause Preterm Birth?
Yes, 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. 1, 2, 3
Mechanism of GBS-Related Preterm Birth
GBS causes preterm birth through ascending infection from the vaginal tract to the amniotic cavity:
- GBS ascends from the vagina to the amniotic fluid after onset of labor or rupture of membranes, and can invade even through intact membranes, triggering preterm labor 1
- The gastrointestinal tract serves as the primary reservoir for GBS, which then colonizes the vaginal tract in approximately 10-30% of pregnant women 1, 4
- Heavy GBS colonization significantly increases the risk of adverse pregnancy outcomes, including preterm birth, stillbirth, and invasive neonatal disease 2, 3
Clinical Evidence Linking GBS to Preterm Birth
The relationship between GBS and preterm delivery is well-established:
- GBS bacteriuria during pregnancy is a marker for heavy genital tract colonization and has been associated with increased risk for preterm birth 1
- Women with GBS bacteriuria who receive additional antibiotics for other indications have a 2.7-fold increased risk of preterm birth (adjusted OR 2.7; 95% CI 1.2-6.1) compared to bacteriuric women not receiving additional antibiotics 5
- Preterm delivery at <37 weeks' gestation is recognized as an important risk factor for early-onset GBS disease, creating a bidirectional relationship where GBS both causes and complicates preterm birth 1
GBS as a Direct Cause vs. Risk Factor
It's critical to understand that GBS functions as both:
- A direct causative agent: GBS infection can trigger preterm labor through inflammatory pathways and ascending infection 2, 3
- A complicating factor: Preterm birth itself increases vulnerability to GBS disease, with mortality rates of 20-30% among infants ≤33 weeks' gestation compared to 2-3% in full-term infants 1
Clinical Management Implications
Because preterm delivery is both a consequence and risk factor for GBS disease, management of threatened preterm delivery requires immediate GBS screening and prophylaxis:
- 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 1
- 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 1
Common Pitfall
The most significant clinical pitfall is failure to initiate GBS prophylaxis in indicated preterm births due to attention being diverted to the medical or fetal complications causing the preterm delivery 6. Always maintain GBS prophylaxis protocols even when managing complex obstetric emergencies requiring preterm delivery 6.