In which situation is intrapartum prophylaxis for Group B Streptococcus (GBS) indicated at 39 weeks gestation with ruptured membranes?

Intrapartum GBS Prophylaxis Indications at 39 Weeks with Ruptured Membranes

Intrapartum prophylaxis for GBS is indicated for the patient with positive GBS bacteriuria treated with oral antibiotics at 20 weeks, as GBS bacteriuria during any trimester of the current pregnancy is an absolute indication for intrapartum prophylaxis regardless of subsequent treatment. 1

Analysis of Each Scenario

1. Positive GBS bacteriuria treated with oral antibiotics at 20 weeks

• This is a clear indication for intrapartum prophylaxis
• The CDC guidelines explicitly state that "GBS bacteriuria during any trimester of the current pregnancy" requires intrapartum antibiotic prophylaxis 1
• Prior treatment with oral antibiotics does not eliminate the need for intrapartum prophylaxis, as oral antibiotics are not effective in eliminating GBS carriage 1, 2
• This patient requires prophylaxis regardless of subsequent negative cultures

2. GBS swab negative 4 weeks ago but membranes ruptured for 18 hours

• Despite the prolonged rupture of membranes (≥18 hours), intrapartum prophylaxis is NOT indicated
• The CDC guidelines clearly state that "negative vaginal and rectal GBS screening culture in late gestation during the current pregnancy, regardless of intrapartum risk factors" is a non-indication for prophylaxis 1
• The negative GBS culture within the past 5 weeks is considered valid and reliable 2

3. Previous pregnancy positive for GBS but current pregnancy negative at 36 weeks

• Intrapartum prophylaxis is NOT indicated
• Prior GBS colonization in a previous pregnancy without a previous GBS-affected infant is not an indication for prophylaxis in the current pregnancy 1, 2
• The negative culture at 36 weeks in the current pregnancy supersedes the history from the previous pregnancy 1

4. GBS positive patient with planned Caesarean Section without ruptured membranes

• Intrapartum prophylaxis is NOT indicated
• The CDC guidelines specifically state that "cesarean delivery performed before onset of labor on a woman with intact amniotic membranes, regardless of GBS colonization status or gestational age" is a non-indication for prophylaxis 1
• The key factors here are: intact membranes and cesarean delivery before labor onset

Important Clinical Considerations

• GBS bacteriuria is a marker for heavy genital tract colonization and increases the risk for early-onset disease in the newborn 1
• Oral antibiotic treatment during pregnancy does not eliminate GBS from the genitourinary and gastrointestinal tracts, and recolonization after treatment is typical 2
• The timing of GBS screening is critical, with 35-37 weeks' gestation providing the best predictive value for colonization status at delivery 2, 3
• For women with ruptured membranes at term, the negative predictive value of a negative GBS screen performed within 5 weeks before presentation is very high 2

Prophylaxis Administration

When indicated, intrapartum antibiotic prophylaxis should consist of:

• Intravenous penicillin G (5 million units initially, then 2.5 million units every 4 hours until delivery) 1
• Ampicillin is an acceptable alternative but penicillin G is preferred due to its narrower spectrum 1
• For penicillin-allergic patients, clindamycin or erythromycin may be used if susceptibility testing has been performed 2

Remember that even short durations of prophylaxis (less than 4 hours) achieve fetal serum levels significantly above the minimal inhibitory concentration for GBS, providing benefit even in precipitous labors 4.