Effects during pregnancy
Chicken pox can be more severe for pregnant women and newborn babies. Pregnant women should avoid exposure to people with chicken pox if possible, although it is relatively rare for a pregnant women to become infected (0.05 - 0.1%). A shingles infection is even more rare.
Women who do experience chicken pox early in their pregnancy have a very low chance of infecting their unborn baby (about 0.4% from conception to 13 weeks and 2.2 % from 13 to 20 weeks). If the baby is infected this can lead to miscarriage, slow growth of the baby and sometimes abnormalities of their bones, muscles, fingers, toes, eyes or brain. Women developing chicken pox after 20 weeks of pregnancy may pass this to their unborn baby, who may experience chicken pox while in the uterus (including a rash, possibly with skin scarring).
Newborn babies are most at risk of developing severe chicken pox if their mother has a rash 4 days before, or 2 days after the birth (up to 30% of babies). However, this is extremely rare (1 in 17,000 pregnancies). If an older baby develops chicken pox, the infection is usually mild. If their mother has immunity, this may protect a fully breastfeeding baby for the first 2 to 3 months of their life, if exposed to chicken pox after birth. Sometimes a first infection during pregnancy can be life-threatening for the woman.
Tests and treatments
Pregnant women who think they may not be immune, and have been exposed to chicken pox, need to have a blood test for IgG and IgM antibodies. Both these will show as 'positive' shortly after being infected, with the IgM often disappearing several months later. The IgG antibody stays positive forever. If you test positive only for IgG, but not IgM, that means your infection happened any time from several months ago, to years in the past. If you test positive for IgG and IgM, then your infection was more recent.