A miscarriage is a loss of a pregnancy. The term is generally used when the loss occurs before the fetus might be able to survive outside the womb, so before about 22 to 24 weeks of gestation. Other terms that are used for such losses include spontaneous abortion and early pregnancy failure.
About 15% to 20% of known pregnancies end in a miscarriage. Most miscarriages happen before the pregnancy is 12 weeks along. In the first several weeks of a pregnancy, a miscarriage sometimes happens because a fertilized egg did not properly form a fetus. In many cases, the activity of the fetal heart stopped days or weeks before the symptoms of miscarriage began.
A common sign of miscarriage is bleeding, although not all pregnancies with bleeding end in miscarriage.
In the years before ultrasound, a miscarriage was diagnosed when a woman's body had expelled all or some of the fetus, the placenta and the fluid surrounding the baby. Today, ultrasound may identify pregnancy loss before the body has begun the process of expulsion. Sometimes this finding is called a "miscarriage."
Miscarriages and possible miscarriages are categorized in several ways:
Threatened miscarriage — A miscarriage is considered threatened or possible, when any bleeding from the uterus occurs before 20 weeks but the cervix is closed and there is evidence of continued fetal heart activity.
Inevitable abortion or miscarriage — A miscarriage is called inevitable, meaning it cannot be stopped, if there is bleeding from the uterus and the cervix is opening prior to 20 weeks, but neither the fetus nor placenta has passed out of the woman's body yet. The membranes around the fetus may or may not have ruptured (broken).
Incomplete abortion or miscarriage — A miscarriage is incomplete when a portion of the fetus or placenta has passed out of the uterus prior to 20 weeks gestation, but some of the placental or fetus remains in the uterus.
Complete miscarriage — A miscarriage is complete if the fetus, all the membranes around the fetus and the placenta are expelled completely and the cervix closes prior to 20 weeks.
Missed abortion or miscarriage — A missed abortion refers to a miscarriage in which the fetus has died prior to 20 weeks gestation, but neither the fetus nor the placenta has been expelled from the uterus.
Recurrent miscarriage — A woman is said to have recurrent miscarriage after three or more miscarriages in a row. Approximately 1% of women experience recurrent miscarriages.
Blighted ovum or anembryonic gestation — This occurs when a gestational sac forms inside the uterus, but no fetus is present after seven weeks.
If a pregnancy ends after 20 weeks, it is not considered a miscarriage. The fetus gets delivered, and if the fetus is not living, the event is called a stillbirth.
Problems with fetal chromosomes account for approximately 50% of all miscarriages. Chromosomes are long strings of DNA, each containing thousands of genes. Genes, in turn provide the instructions for the proteins and other molecules that create, shape and govern our bodies and health.
In most cases, the abnormalities of chromosomes that caused a miscarriage do not reflect any abnormality in either parent. Instead they reflect problems that developed at the time the egg or sperm developed, or at the time when fertilization and early division of the fertilized egg took place. In these case, where the miscarriage is related to chromosomal abnormalities, many think of it as the body's way of ending a pregnancy that is not developing normally.
In some unusual circumstances, miscarriages can happen if there are problems with the internal structure of a woman's uterus or the function of her cervix.
Infections such as rubella (German measles) have been associated with miscarriage. This has led some to wonder if there are other infections that cause early pregnancy loss, but few links have been confirmed. Some experts have also suggested that hormonal imbalances can result in miscarriage, but identifying specific abnormalities has proven difficult.
In rare cases, a woman's immune system appears to react with the pregnancy tissue, resulting in miscarriage. Overall, immune system disorders seem to be responsible for very few miscarriages, but among women who have had three or more miscarriages in a row (recurrent miscarriage), immune-system disorders are responsible for 5% to 10% of these losses. One fairly common antibody problem that can lead to recurrent miscarriage is called "antiphospholipid antibody syndrome." Another example of an immune related issue associated with miscarriage is the production of antibodies that react to the thyroid gland (thyroperoxidase antibodies).
Having either type of antibody is unusual. But it may be more common when there have been several miscarriages (recurrent miscarriage). Testing for such antibodies after several miscarriages is often suggested. Treatment for the antibody disorders may decrease the risk of future miscarriage.
In many cases, no cause for a miscarriage is identified.