The standard advice used to be that there was no risk if you contracted chickenpox after 20 weeks of pregnancy. That has just been updated following a review of the research by Drugs and Therapeutics Bulletin. DTB found there was a risk up until 28 weeks, when it drops to zero.
Fetal varicella syndrome, caused by chickenpox, can be nasty although only a minority of women exposed to chickenpox will be affected. The risk is 0.5 per cent up to 12 weeks, 1.4 per cent 12-28 weeks.
FVS can cause low birth weight, skin loss, scarring, affect the development of limbs and also death. So if you do catch chickenpox after 20 weeks, make sure your doctor takes it seriously and doesn't rely on the old advice that after that stage there is no risk at all.
Public service announcement over!