Chicken Pox (Varicella)*
Most babies born to women who have chicken pox in pregnancy are normal. However, 1-2 percent of the babies whose mothers had chicken pox in pregnancy have one or more birth defects due to the infection. The virus that causes chicken pox infects the fetus 25 percent of the time.
Sometimes, infection of the fetus causes a pattern of birth defects called the varicella embryopathy. The birth defects seen include scars, eye problems, poor growth, underdevelopment of an arm or leg, small head size, delayed development and/or mental retardation. Some babies may have only one of these problems while others have some or all.
The greatest risk for birth defects is when the mother develops chicken pox between 8 and 20 weeks of pregnancy. The risk for birth defects due to chicken pox infection in the first trimester of pregnancy is 0.5-1 percent. When chicken pox occurs between 13 and 20 weeks of pregnancy, the risk for birth defects appears to be 2 percent.