In healthy adults, infection with cytomegalovirus (CMV) can cause mild symptoms (similar to a flu) or may have no symptoms, but if women are infected during pregnancy, the virus can -- rarely -- cause important problems for a developing fetus.

About 50% of women are infected before becoming pregnant. Many don't know they are infected. As a result, they are mostly immune to becoming infected again.

IgG antibodies are evidence of past infections. IgM antibodies suggest more recent infection. Being negative for both means you are susceptible to infection.

Now for the scary part: About 1-2% of women are infected with CMV during pregnancy. Their pregnancies are at risk for problems. These include growth abnormalities, microcephally (small head size) and abnormal brain development.

CMV infection is the most common cause of congenital deafness. In some cases, the infection of a fetus during pregnancy can be identified by ultrasound. But often the infection is only recognized when problems are seen at birth.

Not all babies of mothers who are infected with CMV have problems. The risk of problems remains quite low.

CMV is out there, but those with infections may not recognize it. Because there is little to be done if a mother is identified as being susceptible to CMV, routine testing is not recommended in pregnancy. Many doctors do not test for it. And there is no practical way to tell pregnant women at risk to avoid CMV.