What are the guidelines for intrapartum antibiotic prophylaxis (IAP) in pregnant women at high risk for transmitting Group B Streptococcus (GBS) to their newborn?

Guidelines for Intrapartum Antibiotic Prophylaxis in High-Risk Pregnant Women

Pregnant women at high risk for transmitting GBS to their newborn should receive intrapartum antibiotic prophylaxis based on three absolute indications: previous infant with invasive GBS disease, GBS bacteriuria at any concentration during the current pregnancy, or positive vaginal-rectal GBS screening at 36-37 weeks' gestation. 1, 2

Absolute Indications for IAP (Always Treat)

Category 1: Previous Infant with Invasive GBS Disease

• Any woman with a history of a previous infant with invasive GBS disease requires IAP regardless of current GBS colonization status 1
• No screening is needed in the current pregnancy for this indication 1

Category 2: GBS Bacteriuria During Current Pregnancy

• Any concentration of GBS in urine at any trimester during the current pregnancy mandates both treatment of the UTI and IAP during labor 1, 2
• This applies whether the bacteriuria is symptomatic or asymptomatic 1
• Women with GBS bacteriuria do not need third-trimester screening because they automatically qualify for IAP 1
• The rationale is that GBS bacteriuria indicates heavy genital tract colonization and significantly increases risk of early-onset neonatal disease 1, 2

Category 3: Positive GBS Screening Culture

• All pregnant women should undergo vaginal-rectal GBS screening at 36 0/7 to 37 6/7 weeks' gestation 3, 4
• Women with positive screening cultures require IAP during labor 1
• The updated screening window (36-37 weeks rather than 35-37 weeks) reflects more recent guidance 3

Conditional Indications (Unknown GBS Status Only)

When GBS colonization status is unknown at labor onset, administer IAP if ANY of the following risk factors are present: 1

• Delivery at <37 weeks' gestation 1
• Rupture of membranes ≥18 hours 1
• Intrapartum temperature ≥100.4°F (≥38.0°C) 1
• Positive intrapartum NAAT (nucleic acid amplification test) for GBS 1

Critical Caveat on Risk-Based Approach

If a woman has a negative vaginal-rectal GBS culture at 36-37 weeks, she does NOT require IAP even if intrapartum risk factors develop 1. The negative culture supersedes risk factors.

When IAP is NOT Indicated

Cesarean Delivery Exception

• IAP is not indicated for planned cesarean delivery performed before labor onset with intact amniotic membranes, regardless of GBS status or gestational age 1
• However, these women should still undergo routine GBS screening at 36-37 weeks because labor or membrane rupture may occur before the planned cesarean 1

Historical Colonization

• GBS colonization during a previous pregnancy does not indicate IAP in the current pregnancy unless current pregnancy indications are present 1
• GBS bacteriuria during a previous pregnancy does not indicate IAP in the current pregnancy 1

Recommended Antibiotic Regimens

First-Line: Penicillin G (Preferred)

• Penicillin G 5 million units IV initial dose, then 2.5-3.0 million units IV every 4 hours until delivery 1, 2
• Preferred due to narrow spectrum of activity and universal GBS susceptibility 2

Alternative: Ampicillin

• Ampicillin 2 g IV initial dose, then 1 g IV every 4 hours until delivery 1
• Acceptable alternative but has broader spectrum than penicillin 2

For Non-Severe Penicillin Allergy

• Cefazolin 2 g IV initial dose, then 1 g IV every 8 hours until delivery 1, 2
• Cross-reactivity between penicillins and cephalosporins occurs in approximately 10% of patients 2

For Severe Penicillin Allergy (High Risk for Anaphylaxis)

• Clindamycin 900 mg IV every 8 hours until delivery IF susceptibility testing confirms susceptibility 1, 2
• Susceptibility testing is mandatory because clindamycin resistance is increasing 2, 5
• Erythromycin is no longer acceptable for IAP 1
• Vancomycin should be reserved only when no other options exist 2

Timing and Duration Considerations

Optimal Duration

• IAP should be administered for at least 4 hours before delivery to achieve adequate fetal tissue levels 1, 2, 6
• Recent evidence suggests that even "inadequate" IAP with beta-lactams (less than 4 hours) may be more effective than previously thought 7
• However, the 4-hour threshold remains the official guideline standard 1, 2

Practical Implementation

• Medically necessary obstetric procedures should NOT be delayed to achieve 4 hours of prophylaxis 1
• Procedures to facilitate labor progression (such as amniotomy) should be timed to allow for 4 hours of IAP when possible, but this is not mandatory 1

Critical Pitfalls to Avoid

Do NOT Treat GBS Colonization Before Labor

• Antimicrobial agents should not be used before the intrapartum period to eradicate GBS colonization 1
• Antepartum treatment is ineffective in eliminating carriage, does not prevent neonatal disease, and promotes antibiotic resistance 1, 2
• The exception is treatment of symptomatic or asymptomatic GBS urinary tract infection, which should be treated at diagnosis AND followed by IAP during labor 1

Clindamycin Resistance

• A case report documented clindamycin-resistant GBS disease in a newborn whose mother received clindamycin IAP 5
• This underscores the critical importance of susceptibility testing before using clindamycin 2, 5

Compliance Challenges

• A recent study found that only 61% of women with IAP indications received "adequate" IAP (≥4 hours before delivery) 6
• The most frequent reason for inadequate IAP was less than 4 hours from birth despite spending more than 60 minutes in the hospital 6
• This highlights the need for early recognition and prompt antibiotic administration upon admission 6

Suspected Chorioamnionitis

• If chorioamnionitis is suspected, broad-spectrum antibiotic therapy that includes an agent active against GBS should replace GBS prophylaxis 1
• GBS prophylaxis alone is insufficient for suspected chorioamnionitis 1