Spontaneous abortion: A miscarriage, that is, any pregnancy that is not viable (the fetus cannot survive) or in which the fetus is born before the 20th week of pregnancy. Spontaneous abortion occurs in at least 15-20% of all recognized pregnancies and usually takes place before the 13th week of pregnancy.
A spontaneous abortion is as opposed to an induced abortion. In a spontaneous abortion, it is purely accidental, that is spontaneous.
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