All pregnant women who come into contact with Chicken Pox should immediately contact their GP/Midwife, btw. They can do IgA and IgG tests (blood tests) which show whether you have immunity from past incidence or if you are developing antibodies (ie. recently infected). There is limited treatment if you are shown to be unprotected and have been exposed.
From NHS UK:
Chickenpox during pregnancy can cause complications both for the pregnant woman and the unborn baby. However, the risk of complications is low.
It?s rare to get chickenpox when you?re pregnant. In the UK, it?s estimated that about three 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
you know that you haven?t had chickenpox or you?re not sure, and you?ve been near someone with chickenpox (even if you have no rash or other symptoms)
Also contact your GP or midwife immediately if 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 you:
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 one 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)
heart muscle (myocarditis)
kidneys (glomerulonephritis)
appendix (appendicitis)
pancreas (pancreatitis)
Very rarely, complications from chickenpox during pregnancy can be fatal.
Complications for the unborn baby
Complications that can affect the unborn baby vary, depending on how many weeks pregnant you are when you get chickenpox.
If you catch chickenpox up to 28 weeks of pregnancy, there?s no evidence that it increases your risk of miscarriage. However, there?s a small risk that the baby can develop foetal varicella syndrome (FVS). FVS can damage the baby?s eyes, legs, arms, brain, bladder or bowel. Research has shown that FVS can affect one to two in every 100 babies (1-2%).
If you catch chickenpox between 28 and 36 weeks, 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, when it causes 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 get chickenpox:
around the time of the birth and the baby is born within seven days of your rash developing
up to seven days after giving birth