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What Is a Miscarriage?

A miscarriage, or spontaneous abortion, is a pregnancy that ends by itself within the first 20 weeks. "Stillbirth" refers to the loss of a pregnancy with fetal death when it occurs after the first 20 weeks. Experts estimate that about half of all fertilized eggs die and are miscarried, usually even before the woman knows she is pregnant.

Of pregnancies that the mother knows about (because she has missed her period or her pregnancy has been confirmed by a healthcare provider, or both), approximately 10%-20% end in miscarriage, making miscarriage a very common occurrence. In most cases, miscarriage may be considered a natural-selection process because it is the ending of a pregnancy that would not have developed into a healthy baby. Only placental tissue, not a fetus, had formed.

The term "abortion" is commonly used to refer to the deliberate ending of a pregnancy. But, medically speaking, it refers to both the intentional and the unintentional ending of a pregnancy up until the time a fetus can be expected to survive outside the womb. Hence, healthcare providers commonly use the medical term "spontaneous abortion" to refer to miscarriage.

What Causes a Miscarriage?

In most cases, the specific cause of a miscarriage is not determined. However, when pregnancy tissues from miscarriages are tested, results show that genetic abnormalities cause about 50%-60% of early miscarriages, those before 12 weeks, and likely that the statistics are higher. Uterine defects, such as a weak cervix, account for 10%-15% of miscarriages and they usually happen later in pregnancy. Hormonal imbalances - including progesterone deficiency, high androgen levels, and imbalances caused by autoimmune diseases - account for a small percent, likely around 5% of miscarriages.

Certain health conditions raise your risk. Women at greater risk are those who have a systemic disease such as diabetes, lupus or hypothyroidism; endometriosis; a uterine infection; or fibroids or uterine scar tissue.

An infection such as rubella or toxoplasmosis during pregnancy increases your risk of miscarriage, as does a fever higher than 100.4 degrees Fahrenheit. Miscarriage is more likely in women who have an abnormal uterus or a weak cervix. Less common causes include infections such as chickenpox, chlamydia, and rubella, or fever during pregnancy. Another suspected cause of some miscarriages is an immune response in which the mother's body rejects the father's genetic material, but this theory and proposed treatments are controversial.

Lifestyle factors may cause some miscarriages, including drinking alcohol, smoking and consuming more than 300 mg of caffeine a day, the equivalent of three cups of coffee or tea or more. Malnutrition, cocaine or crack use, and exposure to radiation or toxic substances raise the risk of a miscarriage. Emotional stress after conception may play a role, but more research into this is needed. Contrary to common belief, video display terminals (such as computer monitors) have not been shown to cause miscarriage.

After one miscarriage, you do not have an increased risk in the next pregnancy. Your risk rises if you have had more than one miscarriage in a row.

The majority of miscarriages happen during the first 10-12 weeks of pregnancy. They are more common in women who are older than 35 or who are carrying multiple fetuses. Miscarriage rates for 40-year-old women are about twice that for 20-year-old women, because of the increased risk of chromosomal malformations in older women.

Stages and Types of Miscarriage

There are several stages and types of miscarriages.

• Threatened abortion. When early symptoms of a miscarriage occur such as vaginal bleeding, but usually without other symptoms.
• Inevitable abortion. The membranes have broken or the cervix has dilated too much.
• Incomplete abortion. Some of the pregnancy tissues have been expelled while others remain inside the uterus.
• Complete spontaneous abortion. All of the pregnancy tissues are expelled from the uterus.
• Missed abortion. The fetus has not developed or has died, but no bleeding or other symptoms are observed and pregnancy tissues have not been expelled from the uterus.
• Septic (infected) abortion. A serious infection that occurs in the fetal material before, during or after a miscarriage.