Chickenpox is caused by primary infection with varicella-zoster virus. Shingles is caused by the reactivation of dormant virus in dorsal root ganglion. In pregnancy, there is a risk to both the mother and also the fetus, a syndrome now termed fetal varicella syndrome

Risks to the mother

• 5 times greater risk of pneumonitis

Fetal varicella syndrome (FVS)

• risk of FVS following maternal varicella exposure is around 1% if occurs before 20 weeks gestation
• studies have shown a very small number of cases occurring between 20-28 weeks gestation and none following 28 weeks
• features of FVS include skin scarring, eye defects (microphthalmia), limb hypoplasia, microcephaly and learning disabilities

Other risks to the fetus

• shingles in infancy: 1-2% risk if maternal exposure in the second or third trimester
• severe neonatal varicella: if the mother develops rash between 5 days before and 2 days after birth there is a risk of neonatal varicella, which may be fatal to the newborn child in around 20% of cases

Management of chickenpox exposure in pregnancy, i.e. post-exposure prophylaxis (PEP)

• if there is any doubt about the mother previously having chickenpox maternal blood should be urgently checked for varicella antibodies
• historically, exposure has been managed through the timely administration of varicella zoster immunoglobulin (VZIG). However, the guidance has changed due to a national/international VZIG shortage. This was initially a short-term deviation from practice in 2022 but has now become baked into longer-term guidance
• oral aciclovir (or valaciclovir) is now the first choice of PEP for pregnant women at any stage of pregnancy
  • antivirals should be given at day 7 to day 14 after exposure, not immediately
  • why wait until days 7-14? From the PHE guidelines: 'In a study evaluating the comparative effectiveness of 7 days course of aciclovir given either immediately after exposure or starting at day 7 after exposure to healthy children, the incidence and severity of varicella infection was significantly higher in those given aciclovir immediately (10/13 (77%) who received aciclovir immediately developed clinical varicella compared with 3/14 (21%) who started aciclovir at day 7)'

Management of chickenpox in pregnancy

• if a pregnant woman develops chickenpox in pregnancy then specialist advice should be sought
• there is an increased risk of serious chickenpox infection (i.e. maternal risk) and fetal varicella risk (i.e. fetal risk) balanced against theoretical concerns about the safety of aciclovir in pregnancy
• consensus guidelines (Health Protection Authority and RCOG) suggest oral aciclovir should be given if the pregnant women is ≥ 20 weeks and she presents within 24 hours of onset of the rash
• if the woman is < 20 weeks the aciclovir should be 'considered with caution'