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About 15 percent of known pregnancies end in miscarriage. See more pregnancy pictures.

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A miscarriage is the ending of a pregnancy due to the premature delivery of the fetus before the 20th week of pregnancy, a point at which the fetus is not developed enough to survive outside the uterus on its own. A miscarriage is called a spontaneous abortion in medical terms, but after the 20th week of pregnancy it is called a premature delivery or, if the fetus is born dead, a stillbirth. Most miscarriages, however, occur within the first 14 weeks of pregnancy.

About 15 percent of known pregnancies end in miscarriage, but it is impossible to know how many miscarriages occur during the first month of pregnancy, before many women may even realize they are pregnant. The only indication may be a slightly late menstrual period with a heavier than normal flow.

There are several different types of miscarriages. They include:

  • Threatened miscarriage. One of every five pregnant women experiences this when she bleeds vaginally during the first three months. Although it may indicate that a spontaneous abortion will eventually occur, it is often no more than a threat, and the pregnancy continues normally.
  • Inevitable miscarriage. When a pregnant woman begins to bleed and the cervix dilates, it is then only a matter of time before the contents of the uterus are expelled.
  • Missed miscarriage. In this situation, the fetus dies in the uterus but is not naturally expelled and the woman has no bleeding or pain to signify that the pregnancy is not progressing. The physician usually diagnoses the condition when the uterus stops enlarging.
  • Incomplete miscarriage. This occurs when only part of the uterine contents are naturally expelled.
  • Complete miscarriage. This occurs when all the uterine contents have been naturally expelled.

It's often not clear why a miscarriage occurs, but in many cases it is believed that a fetus is aborted because it is not developing normally. This is thought to be a chance event and is usually not due to a defect in either parent. Several factors can contribute to abnormal fetal development, including:

  • abnormalities in the father's sperm or in the mother's egg
  • disease in the mother, such as rubella (German measles), severe heart or kidney disease, diabetes, or thyroid disease
  • abnormalities in the uterus
  • the mother's use of certain drugs
  • the mother's exposure to toxic substances or certain environmental pollutants

Although women often worry that severe emotional trauma or stress or simply falling can cause a miscarriage, this is rarely--if ever--the case. That's true for automobile accidents, too.

On the next page, learn about the symptoms of a miscarriage.