Chickenpox is a viral disease  that isn't serious for most people. But it can be dangerous to a pregnant woman, who is at risk for more complications than if she weren't pregnant, and her fetus, who may be at greater risk of birth defects. The Nemours Foundation has more information about chicken pox and pregnancy:

If you're pregnant, it's important to avoid contact with anyone who has chickenpox.
An infant is at increased risk of complications if the mother gets chickenpox just before or after delivering the baby.
Having had chickenpox prior to pregnancy offers some protection to the baby, since the mother's immunity is passed to the infant via the placenta, and later, breast milk.