Fifth disease, or erythema infectiosum, is a viral infection that is caused by a human parvovirus (B19). This virus, which is transmitted mainly by body fluids, is one that lives only in humans.

"It's only a serious concern in pregnant women during the first trimester," says Dr. Daniel R. Bronfin, section head of general academic pediatrics at the Oschner Clinic in New Orleans, La.

Dr. Vinay N. Reddy of Bloomfield, Mich., explains why it is more serious if contracted during early pregnancy. "If a woman is infected during pregnancy, the baby can develop fetal hydrops," says Dr. Reddy. "This is a condition where the baby's blood cells break down faster than they are made. Some babies – less than 10 percent of those whose mothers were infected in the first half of pregnancy – die of hydrops."

According to the New York State Department of Health, the infection in pregnant women has also been known to increase the risk of miscarriage or spontaneous abortion.

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