Teratogenicity and Treatment of Varicella in Pregnancy
Risk of Teratogenicity
Varicella infection in pregnancy carries approximately a 3% risk of congenital varicella syndrome when infection occurs during the first 20 weeks of gestation, with the highest risk between 13-20 weeks. 1
Timing-Specific Fetal Risks
First and second trimester (before 20 weeks): Congenital varicella syndrome occurs in approximately 3% of cases, manifesting as limb hypoplasia, skin scarring, eye abnormalities, and neurological defects 1, 2
Peripartum period (5 days before to 2 days after delivery): This represents the highest-risk window for severe neonatal disease with historical mortality rates reaching 31% without intervention 3
After 20 weeks (excluding peripartum): Risk of congenital syndrome is minimal, though vertical transmission can still occur at approximately 25% 2
Treatment Algorithm for Pregnant Women
Post-Exposure Prophylaxis (Non-Immune Pregnant Women)
Administer varicella-zoster immune globulin (VZIG) within 96 hours of exposure to prevent severe maternal complications, as this is the primary indication rather than fetal protection. 4, 1
VZIG reduces infection rates from >70% to approximately 30% in exposed non-immune pregnant women 4
Critical caveat: VZIG does NOT prevent viremia, fetal infection, or congenital varicella syndrome—its purpose is solely to reduce maternal disease severity 4, 5
If VZIG is unavailable or >96 hours have elapsed since exposure, oral acyclovir prophylaxis (800 mg five times daily for 7 days starting 7-10 days post-exposure) should be considered 1, 6
Recent evidence suggests oral acyclovir may be equally effective as VZIG, with infection rates of 30.8% versus 36.6% respectively 6
Treatment of Active Varicella Infection in Pregnancy
For pregnant women who develop varicella, initiate oral acyclovir 800 mg five times daily within 24 hours of rash onset to reduce maternal complications. 5, 1
Acyclovir is FDA Pregnancy Category B with excellent safety data: a registry of 749 first-trimester exposures showed no increased rate of birth defects compared to the general population 7, 5
Escalate to intravenous acyclovir (10-15 mg/kg or 500 mg/m² every 8 hours) for:
Treatment effectiveness decreases significantly if started >24 hours after rash onset 8
Monitoring and Fetal Assessment
Perform detailed ultrasound examination for all pregnant women who develop varicella to screen for fetal consequences, particularly if infection occurred before 20 weeks 1
Serial ultrasounds should assess for limb abnormalities, microcephaly, hydrocephalus, and growth restriction 1
Neonatal Management for Peripartum Maternal Infection
Neonates born to mothers with varicella onset from 5 days before to 2 days after delivery must receive VZIG immediately after birth (125 units/10 kg, maximum 625 units), regardless of whether the mother received VZIG. 4, 3, 1
Despite VZIG prophylaxis, approximately 62% of these high-risk neonates will still develop varicella, but complications and mortality are substantially reduced 4, 3
If varicella develops despite VZIG: Initiate intravenous acyclovir immediately (10 mg/kg IV every 8 hours for 10 days) 3
Neonates whose mothers developed varicella >5 days before delivery are protected by transplacentally acquired maternal antibody and do not require VZIG 4
Critical Pitfalls to Avoid
Do not delay VZIG administration beyond 96 hours post-exposure—effectiveness decreases significantly after this window 9, 1
Do not assume VZIG prevents fetal infection—counsel patients that it only reduces maternal disease severity, not congenital syndrome risk 4, 5
Do not withhold acyclovir due to pregnancy concerns—the safety profile is excellent with no teratogenic risk demonstrated 7, 5
Do not miss the 24-hour treatment window for acyclovir initiation after rash onset—efficacy drops substantially thereafter 8
Do not forget neonatal VZIG even if the mother received it—neonates require their own dose when maternal rash occurs in the critical peripartum window 4, 3