According to WebMD, the early signs of a miscarriage include vaginal bleeding, pain and blood clots coming from the vagina. While first-trimester vaginal bleeding is common during a normal pregnancy, bleeding with pain may indicate a miscarriage. Pelvic cramps, belly pain and a lower backache may also indicate a possible miscarriage. The American Pregnancy Association points out that weight loss and contractions are also possible signs of miscarriage.
WebMD notes that not all miscarriages are alike, and one woman's experience of a miscarriage may differ greatly from another woman's experience. Some miscarriages occur in stages spread across several days. Even healthy women stand a 15 to 20 percent chance of having a miscarriage when pregnant, according to the American Pregnancy Association.
WebMD lists risk factors for miscarriage including infections during pregnancy, a history of miscarriage, exposure to certain chemicals or medicines, blood-clotting disorders, polycystic ovary syndrome and problems with the uterine structure. Use of alcohol, tobacco, cocaine or large amounts of caffeine also increase the risk of miscarriage. In addition, the risk of miscarriage is greater when either the mother or father of the baby is over 35 years old.
Most miscarriages occur, however, not because of anything the mother did during pregnancy but because of problems in the development of the fetus, according to WebMD. Exercise, sex and stress do not cause miscarriages.