Doctors generally define a miscarriage as the natural termination of a pregnancy during the first 20 weeks of gestation. (Delivery between the 21st and 38th weeks is considered a premature birth, even if the fetus does not survive.) At least 20 percent of all pregnancies end in a miscarriage between the sixth and tenth weeks. Recent studies indicate, however, that the figure may be as high as SO percent, but that many go unnoticed because women don’t always realize they are pregnant. Typically, a miscarriage begins with aginal bleeding or a brownish discharge, which may be accompanied by cramps and lower back pain. If the woman is unaware of being pregnant, she may simply assume that she is having a heavy menstrual period. The majority of miscarriages are caused by a problem in fetal development, most often chromosomal abnormalities or structural malformations. For example, fertilization toward the end of the 24 hour period after ovulation can cause these abnormalities. Some researchers believe late fertilization is the major cause of miscarriage. Other possible causes include maternal hormone imbalances, such as those caused by thyroid disease or diabetes a. structural defect in the uterus or ervix; poor attachment of the placenta to the uterine wall; rubella or other infectious illnesses, including sexually transmitted diseases; complications from kidney disease; the presence of an IUD in the uterus; or severe emotional shock. Lifestyle factors that increase the risk of miscarriage include smoking, use of alcohol, poor nutrition, and exposure to radiation, hazardous chemicals, or lead. For many miscarriages, however, a cause cannot be found.