A miscarriage is the spontaneous loss of a pregnancy from conception to 20 weeks' gestation. Miscarriage is sometimes referred to as spontaneous abortion, because the medical term abortion means the ending of a pregnancy, whether intentional or unintentional. Most miscarriages occur in the first trimester of pregnancy, from seven to twelve weeks after conception.About 10 to 20 percent of known pregnancies end in miscarriage. But the actual number is likely higher because many miscarriages occur so early in pregnancy that a woman doesn't realize she's pregnant
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http://rbej.biomedcentral.com/articles/10.1186/1477-7827-2-7. biomed central, methylenetetrahydrofolatereductase and transcobalamin genetic polymorphisms in human spontaneous abortion: biological and clinical implications, reproductive biology and endocrinology20042:7, doi: 10.1186/1477-7827-2-7, zetterberg