Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Miscarriage

What Is It?

A miscarriage is the loss of a pregnancy before 20 weeks, the point at which a fetus might be able to survive outside the womb. Miscarriage also is called spontaneous abortion or natural abortion. In a miscarriage, the woman's body expels all or some of the fetus, the placenta and the fluid surrounding the baby.

About 15% to 20% of known pregnancies end in a miscarriage, and many more happen before a woman recognizes she is pregnant. Most miscarriages happen before the pregnancy is 16 weeks along. In many cases, the fetus died days or weeks before the symptoms of miscarriage began. In the first several weeks of a pregnancy, a miscarriage sometimes happens because an embryo did not properly form into a fetus, a situation known as a blighted ovum.

Miscarriage most often happens when the fetus has abnormal chromosomes, the pieces of DNA that contain genes and determine what we are like. Chromosome problems account for approximately 50% of all miscarriages. About 95% of the time, the parents' chromosomes are normal and the genetic abnormality has developed as a one-time error in the fetus. In such cases, miscarriage is the body's way of ending a pregnancy that is not developing normally.

Miscarriages also can happen if there are problems with the internal structure of a woman's uterus or the strength of her cervix. For example, some women are born with a uterus that has a thick dividing septum extending through its middle. This can nearly or completely divide the womb into two chambers, and a uterus with this shape may not hold a pregnancy securely. A woman's cervix, which should open during labor and delivery to allow the fetus to pass through, is sometimes too weak to keep the fetus safe inside the womb until delivery. When the cervix is too weak and opens early in pregnancy it is called an incompetent cervix. Problems with the structure of the uterus and the problem of an incompetent cervix are the two most common causes of miscarriage during the first part of the second trimester (12 to 20 weeks).

Other causes of miscarriage include infections and hormonal imbalances. In rare cases, a woman's immune system rejects the fetus, or antibodies from the immune system may cause problems with placenta blood flow. Immune-system disorders are responsible for miscarriages in 5% to 10% of women who have had three or more miscarriages in a row. One fairly common antibody problem that can lead to miscarriage is called "antiphospholipid antibody syndrome."

Women with diseases such as poorly controlled diabetes or severe hypothyroidism (underactive thyroid) are at higher risk of miscarriage. Having rubella (German measles) during early pregnancy has been associated with miscarriage. Environmental factors, such as smoking cigarettes and drinking alcoholic beverages during pregnancy, may increase your risk of having a miscarriage. Miscarriage is not caused by regular physical activities, minor accidents, exercise, sexual intercourse, or minor stumbles or falls.

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 the fetus is alive.

• 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 have passed out of the woman's body yet. The membranes around the fetus may or 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. It is a delivery, and if the fetus is not living the event is called a stillbirth.