By the time you become pregnant, you are likely immune to chickenpox,  you’ve probably already have been exposed to the chickenpox virus or you have been immunized. Most people in the United States get immunized against chcikenpox or had chickenpox when they’re a child. After you’ve had the infection once, and after you have been immunized, you’re likely immune for life. More than 95% of adults are immune against chickenpox, but there are a few people who are not immune. An estimated seven in every 10,000 pregnant women will get chickenpox. If you’re not sure whether or not you’re immune, you can get a quick blood test to find out. 

The best time to get the blood test to see if you are immune is before you are pregnant, so you can get immunized. Once you are pregnant it's too late to get immunized because the vaccine is with the live attenuated virus.

If you’re not immune to chickenpox when you become pregnant, you should be careful to avoid getting the virus during your pregnancy. Since your baby is developing rapidly in the first and second trimesters of your pregnancy, your chickenpox could cause problems with his or her development.

However, chickenpox in the beginning of the third trimester probably won’t have any ill effect on your baby. There is a chance that your baby could contract congenital varicella syndrome (CVS), which could cause a number of birth defects. Malformed limbs, scarring of the skin, problems with eyesight and an abnormal head size are all birth defects associated with CVS. Luckily, CVS is rare, even in women who contract chickenpox in the first or second trimester. The worst time for you to get chickenpox during your pregnancy is in the five days leading up to your birth or the two days post delivery. While your baby is in the womb, your antibodies will protect both you and the fetus. Your baby might contract part of the virus with you, but your immune system will take care of it. Once your baby is out of the womb, your antibodies will not help, and your newborn’s immune system has barely developed.

Because his or her body will not be able to fight the infection, the condition could become life threatening. If your due date is approaching and you’re not immune, you should take extra precaution to avoid coming into contact with anyone who has chickenpox. It is easily spread through oral germs and blister contact. If you’re not immune to chickenpox when you become pregnant, your doctor will give you a blood test to make sure. Then, you might receive a special short-term vaccination called VariZIG to lessen the possibility of contracting it during your pregnancy, and you should take extra care to avoid anyone that comes down with chickenpox.

Source: Alon Shrim et al: Management of Varicella Infection (Chickenpox) in Pregnancy. SOGC Clinical Practice Guideline Volume 274 March 2012