Its most dangerous within the first 4 weeks. Found this on a NHS website for you
Testing
If you are pregnant, breastfeeding or have a weakened immune system you are more at risk of developing complications. Babies less than four weeks of age are also at risk. If you fall into one of these groups and have been in contact with someone with chickenpox, or you suspect you, or your child may have developed the condition, then you should visit your GP straight away. You may require some tests to establish your immunity to the condition.