Here's some info from NHS choices
Chickenpox during pregnancy can cause complications, both for the pregnant woman and the unborn baby. However, the actual risk of any complications occurring is low.
It’s rare to get chickenpox when you’re pregnant. In the UK, it’s estimated that just 3 in every 1,000 women (0.3%) catch chickenpox during pregnancy.
Most pregnant women who get chickenpox recover, with no adverse effects on the baby.
When to get medical advice
Seek advice from your GP or midwife immediately if you’re pregnant and:
you think you may have chickenpox
either you've never had chickenpox or you’re not sure, and you've been near someone that has it (even if you have no rash or other symptoms)
you get chickenpox within seven days of giving birth
Complications for pregnant women
You have a higher risk of complications from chickenpox if you’re pregnant and:
smoke
have a lung condition, such as bronchitis or emphysema
are taking or have taken steroids during the previous three months
are more than 20 weeks pregnant
Up to 1 in 10 pregnant women with chickenpox can develop pneumonia (inflammation of the lungs). Other rare complications include inflammation of other parts of the body, such as the:
brain (encephalitis)
liver (hepatitis)
Complications that arise from catching chickenpox during pregnancy can be fatal. However, with antiviral therapy and improved intensive care, this is very rare.
Complications for the unborn baby
Complications that can affect the unborn baby vary, depending on how many weeks pregnant you are when you catch chickenpox.
There’s no evidence that catching chickenpox up to 28 weeks into pregnancy increases your risk of suffering a miscarriage. However, there’s a small risk that the baby can develop foetal varicella syndrome (FVS). FVS can damage the baby’s skin, eyes, legs, arms, brain, bladder or bowel.
If you catch chickenpox between weeks 28 and 36 of pregnancy, the virus stays in the baby's body but doesn't cause any symptoms. However, it may become active again in the first few years of the baby’s life, causing shingles.
If you catch chickenpox after 36 weeks, the baby may be infected and could be born with chickenpox.
Complications for the newborn baby
The baby may develop severe chickenpox and will need treatment if you catch it:
around the time of birth and the baby is born within seven days of your rash developing
up to seven days after giving birth
For more information about treatment, see How is chickenpox treated during pregnancy?
Read more information about the complications of chickenpox during pregnancy and questions about pregnancy.
www.nhs.uk/chq/Pages/1109.aspx?CategoryID=54