According to MedlinePlus, the majority of miscarriages occur within the first seven weeks of pregnancy, before the fetus has a heartbeat. BabyCenter notes that more than 80 percent of pregnancies end before 12 weeks.
About half of all fertilized eggs spontaneously abort before pregnancy is even recognized due to a missed period. According to WebMD, of the pregnancies that get as far as the 12th week, 85 percent are carried to term. After the 20th week of gestation, the risk of miscarriage drops further. BabyCenter describes 50 to 70 percent of all first trimester miscarriages as occurring because of chromosomal abnormalities preventing the early embryo from developing into a baby. Age, drug use and family history increase the risk of miscarriage regardless of the week.