My daughter-in-law is four weeks pregnant. Her young daughter has chickenpox. Could this harm the new baby?

H.C.

There are many important issues here. Chickenpox, also known as varicella, is a viral infection that usually affects children. Although it is typically a mild and harmless illness, it can occasionally be severe. Chickenpox can also cause problems in pregnancy, but not as often as other pregnancy-related infections such as toxoplasmosis, syphilis and rubella (German measles).

Chickenpox in adults is often more severe, with more skin blisters or "pox" and a higher risk of pneumonia and other complications. Pregnant women are at particular risk of severe chickenpox. Most of the increased risk occurs in the second or third trimester of pregnancy. Doctors are often taught that chickenpox in a pregnant woman is more harmful to the mother than to the fetus. Although this is probably true, there is still some risk to the fetus. About 1 percent of cases of chickenpox early in pregnancy harm the baby. This syndrome, termed "varicella embryopathy," can involve brain damage, eye problems and limb problems. Because the risk of having any fetal problems is low but the birth defects that do occur can be severe, the woman may face a difficult decision over whether to continue the pregnancy.

In your family's case, I would first try to determine with certainty that your granddaughter indeed has the chickenpox. Usually it is obvious, but other infections can sometimes be confused with this disease. The child's doctor can confirm the infection with a blood test for antibodies to the virus or by culturing (attempting to grow) the virus from your granddaughter's skin blisters.

Even if the child does indeed have chickenpox, your daughter-in-law will be at little to no risk if she has ever had chickenpox before. It would be extremely unusual to catch the disease a second time. If she does not recall being sick with chickenpox, a blood test can determine whether she has antibodies to the virus. If so, she is immune to chickenpox, and she has nothing to worry about. If not, then she is at risk.

If it is still only a day or two since she was exposed to her daughter's disease, then the infection can be prevented. Her doctor can give her an injection of antibody directed against the varicella virus. The injection, called varicella-zoster immune globulin (VZIG), is completely safe and will usually prevent infection. If it's been more than a few days since your daughter was exposed, it is probably too late for the injection, and she is stuck with waiting to see if she gets sick. The incubation period for this infection is from 10 to 21 days, so if she is going to get chickenpox, she'll do so during that time period. After 21 days, she is in the clear, unless she is exposed again. If she does become ill, then she will have a difficult decision to make.