Miscarriages are common. About one of every five or six pregnancies ends in miscarriage, usually in the first few months. Some women experience repeated miscarriages, which can be caused by subtle genetic abnormalities in the couple's chromosomes, problems with the uterine cavity (fibroids, scar tissue, developmental defects), or other disorders. All women who have three or more miscarriages should be tested, especially if the miscarriages are consecutive.

More than half of all miscarriages during the first 13 weeks of pregnancy are caused by problems in the fetus' chromosomes. Chromosomes are tiny structures inside body cells that carry the basic identity of heredity. Each chromosome contains genes that determine a person's appearance, sex, and blood type. Problems in the number or structure of the chromosomes or the genes can lead to miscarriage. Frequently this is nature's way of ending a pregnancy in which the fetus was not developing normally.

Most chromosomal problems occur by chance and are not likely to recur in later pregnancies. But in a small number of cases chromosomal problems can cause repeated miscarriage. A karyotype is a special test to analyze chromosome structure.

Many abnormalities of the uterus, which are linked to miscarriage, can be treated with surgery. A special X-ray (hysterosalpingogram) can detect abnormalities of the uterine cavity, and an endometrial biopsy can provide information about the uterine lining, where implantation occurs.

In some cases the mother's illness has been linked to miscarriage. Systemic lupus erythematosus and other autoimmune disorders, congenital heart disease, severe kidney disease with high blood pressure, uncontrolled diabetes, thyroid disease or an intrauterine infection interferes with pregnancy. When these illnesses are treated, the chances for a successful pregnancy improve. Other illnesses may require close monitoring during pregnancy.

Disorders of the immune system can also lead to miscarriage. The immune system defends the body against disease by recognizing and attacking foreign substances. The mother's body normally protects the fetus from an attack by her own antibodies, but in some cases this protection may be absent in a woman's blood. Problems in the immune system can be diagnosed with a blood test.

Hormone imbalance may also lead to repeated miscarriage. The hormone progesterone prepares the lining of the uterus for the fertilized egg during the second half of the menstrual cycle. When an egg is fertilized, this hormone continues to prepare the uterine lining for the embryo. Enough progesterone must be made to maintain the pregnancy, otherwise miscarriage will occur. If tests indicate that a woman's body is not producing enough progesterone, supplements can be prescribed to correct the problem.

In some cases environmental and lifestyle factors lead to greater risk of miscarriage. Women who smoke, drink heavily or use illegal drugs, especially cocaine, increase their risk of miscarriage. Exposure to high levels of radiation or toxic substances may also be a factor in repeated miscarriage.

Even if you have had repeated miscarriages, you still have a good chance for a successful pregnancy. Your physician will need a great deal of information to determine the cause. You will be asked about your medical history, past pregnancies, lifestyle and work environment.

It's important to have a complete medical workup before you attempt pregnancy again because many of the causes of miscarriage can be treated. If you think you might be pregnant, see your physician right away. You may be referred to a perinatalogist for care.