How Is Parvovirus B19 Diagnosed in the Mother?

Parvovirus is usually diagnosed through a blood test for antibodies to the virus. Antibodies are cells that your immune system produces in response to an infection. If the blood test shows that you have antibodies, you are immune to the virus. If you are exposed to parvovirus during your pregnancy, you should immediately have an antibody test.

Table 1 describes how your doctor interprets the results of antibody tests for parvovirus. Keep in mind that IgM antibody is the first to appear in the course of infection. It usually remains present for 90 to 120 days, then disappears. IgG antibody typically appears seven to 14 days after exposure and remains in the blood for life. A negative test means the antibody is not present; a positive test means it is present.

Table 1. Interpretation of Antibody Tests for Parvovirus - Initial Test Performed As Soon As Possible After Exposure.
Antibody in the Mother Interpretation
IgMIgG 
NegativePositiveIMMUNE-no risk of second infection; no risk of fetal injury
NegativeNegativeSUSCEPTIBLE-test should be repeated in 3 weeks to determine if antibodies appear
PositiveNegativeACUTE INFECTION-infection occurred at least 3, but less than 7, days ago; fetus is at risk and requires monitoring
PositivePositiveSUBACUTE INFECTION-infection occurred more than 7, but less than 120, days ago; fetus is at risk and requires careful evaluation

As you can see, if only the IgG antibody is present, you are immune to the virus. A future infection is extremely unlikely, and your baby is not at risk. However, the presence of the IgM antibody, with or without the IgG antibody, indicates an infection. Your baby is in danger of infection and should be evaluated immediately.

If neither the IgM nor the IgG antibody is present, you are susceptible to infection. Your antibody test should be repeated in about three weeks to determine if an infection has actually occurred. If the IgM antibody appears in your next blood test, your doctor will perform a series of ultrasound examinations over the ensuing eight to 10 weeks to evaluate your baby's well-being.

How Is Parvovirus Infection Diagnosed in the Fetus?

An ultrasound test is the most effective way for your doctor to diagnose parvovirus in unborn babies. The incubation period of the virus-the time between when the virus is transmitted and when symptoms develop-may be longer in a fetus than in a child or adult. So, you should have a series of ultrasound examinations for eight to 10 weeks after your acute (primary) infection. An ultrasound can detect evidence of fetal anemia, the main consequence of fetal infection. Signs of anemia include hydrop (fluid collections in the scalp, under the skin, and in the chest and abdomen) or changes in blood flow patterns (which can be detected by Doppler ultrasound).

If the ultrasound does not show that your baby has hydrops, additional diagnostic studies are unnecessary. However, if the ultrasound does suggest signs of fetal hydrops, and you are less than 15 to 20 weeks pregnant, your doctor will immediately treat your baby.