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Yes, the virus that causes fifth disease is dangerous during pregnancy. It is a teratogen: an agent that can disturb the development of the embryo or, later, the fetus. Teratogens halt the pregnancy or produce a congenital malformation (a birth defect). Classes of teratogens include radiation, chemicals, drugs, and maternal infections.
The virus of fifth disease can infect the fetus. Although no congenital malformations (birth defects) have been reported, fifth disease can halt the pregnancy. It does so by causing the death of the fetus resulting in a miscarriage (a spontaneous abortion). The risk of this happening -- the risk of fetal death from fifth disease --is 5-10%.
You should avoid contact with anyone with fifth disease if you are pregnant.