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).