From Google it seems the risk is very small but up to 20 weeks...
Risks to the Baby
Firstly, if the mother has definitely had chicken-pox, there is no risk to the baby or her. In any case, even if she doesn't recall having had chicken pox, it is likely she has antibodies (80% do) from a silent (asymptomatic) infection. So if there's any doubt about it, she should see a doctor for a blood test to check if she's immune.
Risks to the baby are important at the following two times, if mum gets chicken-pox:
- Before 20 weeks - risk of chicken pox syndrome. This is actually quite rare. Up until 14 weeks the risk is about 0.4%, whereas between 14-20 weeks it is 2%. If a woman has VZ-Ig treatment (see below) after being exposed, the risk is even lower.
- Mum's rash developing within a week before delivery to a month afterwards. It takes about a week for mum to pass the protective antibodies to the baby, so if born before that time, the babe is at risk of overwhelming infection after birth.
Between 20 weeks and term there is no risk to the baby. The other big risk of chicken pox is to the mother. She is much more likely, during pregnancy, to get a chicken pox pneumonia (10%) which can be very severe (even life-threatening).
Getting Treatment
In cases of 'at risk' exposure it is important to give Varicella-Zoster Ig ('man-made' antibodies) to mum and/or baby. This can be lifesaving and significantly reduce the disease severity. It must be given not later than 10 days after exposure.
Remember all of the above ONLY apply if at-risk - ie mum proven not to have antibodies. If in doubt, see your doctor who can do the blood test.