Ideal GBS Prophylaxis for Preterm Labor Before 34 Weeks
For women in preterm labor before 34 weeks with unknown GBS status, initiate intravenous penicillin G or ampicillin immediately at hospital admission and continue until delivery or until preterm labor is ruled out. 1, 2
Immediate Management Algorithm
Step 1: Initiate Prophylaxis at Admission
- Begin IV antibiotics immediately upon hospital admission for any woman presenting with signs and symptoms of preterm labor before 37 weeks gestation with unknown or positive GBS status 1, 2
- Do not wait for GBS screening results before starting prophylaxis in this high-risk scenario 1, 2
Step 2: Antibiotic Selection
For women without penicillin allergy:
- Penicillin G: 5 million units IV initially, then 2.5-3.0 million units IV every 4 hours until delivery 2, 3
- Ampicillin (acceptable alternative): 2 g IV initially, then 1 g IV every 4 hours until delivery 2, 3
For women with low-risk penicillin allergy (no history of anaphylaxis, angioedema, respiratory distress, or urticaria):
For women with high-risk penicillin allergy:
- Clindamycin: 900 mg IV every 8 hours until delivery (only if GBS isolate is confirmed susceptible to both clindamycin and erythromycin) 2, 3
- Vancomycin: 1 g IV every 12 hours until delivery (if susceptibility unknown or isolate resistant to clindamycin) 2, 3
Step 3: Obtain GBS Screening
- Collect vaginal-rectal GBS culture at hospital admission unless a valid negative culture was obtained within the preceding 5 weeks 4
- A single swab should be collected first from the lower vagina, then inserted through the anal sphincter into the rectum 4
Step 4: Adjust Based on Clinical Course
If patient is NOT in true labor:
If GBS culture returns negative:
If GBS culture returns positive OR patient has GBS bacteriuria:
- Continue prophylaxis through delivery 2, 3
- Patient will require intrapartum prophylaxis for any future labor episodes during this pregnancy 2, 3
Special Consideration: Preterm Premature Rupture of Membranes (PPROM)
- For women with PPROM at ≥24 weeks, ampicillin 2 g IV once, followed by 1 g IV every 6 hours for at least 48 hours serves dual purposes: latency prolongation AND adequate GBS prophylaxis 2
- This regimen eliminates the need for separate GBS prophylaxis dosing in the PPROM setting 2
Critical Timing for Maximum Effectiveness
- Prophylaxis administered ≥4 hours before delivery achieves 78-89% reduction in early-onset neonatal GBS disease 2, 5
- However, do not delay medically necessary obstetric interventions solely to reach the 4-hour threshold 2
- Even shorter durations provide some protection, so start antibiotics as soon as labor is recognized 2
Common Pitfalls to Avoid
- Never treat asymptomatic GBS colonization with oral antibiotics before labor – this approach is completely ineffective at eliminating carriage, does not prevent neonatal disease, and promotes antibiotic resistance 1, 2, 4
- Do not assume a negative GBS screen from a previous pregnancy is protective – each pregnancy requires its own screening 1
- Remember that GBS screening is only valid for 5 weeks – women who present in preterm labor more than 5 weeks after a negative screen require repeat testing and prophylaxis while awaiting results 4, 3
- For women with GBS bacteriuria at any point during pregnancy, they automatically qualify for intrapartum prophylaxis regardless of subsequent screening results or treatment of the UTI 2
Why This Approach for Preterm Labor <34 Weeks
Preterm infants face twice the risk of early-onset GBS disease compared to term infants, even with adequate prophylaxis 5. The preterm population accounts for 50-75% of early neonatal GBS infections despite representing a much smaller proportion of births 6. Given the high stakes and the difficulty in distinguishing true preterm labor from false labor at initial presentation, the CDC guidelines explicitly recommend immediate prophylaxis for all women presenting with preterm labor symptoms before 37 weeks with unknown or positive GBS status 1, 2.