Miscarriage: Causes of Miscarriage

Causes of Miscarriage

In general, miscarriage is more common in women over 35 years old and in pregnancies involving more than one fetus. In some multiple pregnancies (twins, triplets, or more), one or more of the fetuses survives even after another one dies. The dead fetus leaves the mother's body when the surviving baby is born.

Some of the factors discussed in this section are more common after repeated—that is, 3 or more—miscarriages. About 1 in 200 women has repeated miscarriages, which physicians call recurrent spontaneous abortion. In many cases—perhaps most—even these miscarriages happen by chance and do not signal a problem in either or both partners. Often no cause is found.

Chromosomal abnormalities.

Problems in the chromosomes of the embryo, by far the most common reason for loss of pregnancy, are found in more than half of miscarriages occurring in the first 13 weeks. Miscarriages apparently eliminate about 95 percent of fertilized eggs or embryos with genetic problems—perhaps nature's way of ending a pregnancy in which the child would be unable to survive. Spontaneous abortions of this type usually occur before the woman knows that she is pregnant. Most chromosomal problems happen by chance, have nothing to do with the parents, and are unlikely to recur.

Sometimes, however, chromosomal abnormalities are caused by the parents' genes. This is more likely if the woman has had repeated miscarriages or if either parent has relatives or a child with birth defects. Genetic testing and analysis of fetal material from the miscarriage can help the doctor identify the problem.

Illness

Miscarriages are much less common in the third trimester. Those that occur are more likely to be due to maternal factors, such as an illness in the mother, than to genetic abnormalities in the baby.

Women with poorly controlled diabetes are at great risk for miscarriage. Those whose diabetes is controlled, however, whether it existed before the pregnancy or developed after conception (gestational diabetes), are no more likely to lose a pregnancy than other women. A woman may not know that she has diabetes, however, until it is discovered during a search for the cause of repeated miscarriages. The routine blood and urine tests given during pregnancy are an effort to identify this problem while it still can be remedied.

Other diseases and conditions linked to increased risk of miscarriage include systemic lupus erythematosus (SLE, or lupus), high blood pressure, and certain infections, such as rubella (German measles), herpes simplex, and chlamydia. Experts disagree about the role of hypothyroidism, or an underactive thyroid gland, in miscarriage, but it's likely that a severe case increases the risk.

With conditions such as diabetes, treating or controlling the problem can improve the odds of a successful pregnancy. Special monitoring may also be required.

Hormone Imbalance

Some women do not make enough progesterone, the hormone that prepares the lining of the uterus to nourish a fertilized egg; and if the uterine lining cannot sustain an egg, miscarriage will occur. Progesterone supplements, given by injection or in vaginal or rectal suppositories, can correct this problem. The medication also can make it more difficult for a dead fetus to be expelled. A blood test and a biopsy of a small amount of tissue taken from the uterine lining can determine whether you are producing enough progesterone naturally. Hormone imbalance also can be caused by diabetes mellitus or thyroid disease.

Abnormalities of the Uterus and Cervix

Anything physically wrong with the uterus or cervix can lead to a miscarriage. Some defects may be present from birth. Fibroids—noncancerous growths made of uterine muscle tissue—can also be at fault. So can a weak cervix that widens too early in pregnancy without any warning signs of labor, releasing the fetus from the uterus.

These physical problems account for up to 15 percent of repeated miscarriages. To diagnose such problems, the doctor may inject the cavity of the uterus with some fluid, then take an x­ray of your uterus and fallopian tubes. Another technique is to examine the inside of your uterus through a long, thin instrument (hysteroscope) inserted through the vagina and cervix. In another procedure, the doctor may make a small incision in the lower abdomen and insert a laparoscope, through which he or she can inspect the pelvic organs. Surgery can correct many abnormalities in the uterus but your doctor probably won't recommend it until all other causes of miscarriage have been ruled out. After surgery, 70 to 90 percent of pregnancies are successful.

Though a weak cervix is a relatively rare condition, it's almost impossible to detect before it becomes apparent during pregnancy, usually after the 15th week. Once discovered, it is likely to disrupt every pregnancy. To remedy the problem, after the first trimester, but before the cervix has dilated (widened) to a certain point, your doctor can reinforce the cervix with sutures, which will be removed when the baby reaches term. Women with bleeding, uterine contractions, or ruptured membranes should not undergo this procedure.

Immune System Problems

A developing baby is half made up of foreign genetic material from the father. Some women have repeated miscarriages because their bodies see each baby as an invading organism and attack it with antibodies. Ordinarily, many elements of the immune system work together to ensure that the mother's body does not reject the baby. But when this coordination fails, a miscarriage follows. Treatments for such problems in the immune system are experimental and should not be tried until other causes for repeated miscarriage have been ruled out. Some research centers have tried to “immunize” the mother with the father's white blood cells, but so far without good results.

Certain autoimmune diseases and abnormalities also increase the risk of miscarriage. Women whose blood contains certain types of antibodies are at particularly high risk. These women may have no symptoms other than trouble retaining a pregnancy, but a blood test can determine whether the antibodies are present. If so, heparin, prednisone, and aspirin during pregnancy can help prevent miscarriage. About 70 to 75 percent of women with lupus-associated antibodies who are treated with these drugs are able to deliver. In any case, if you have these blood abnormalities, you should have your doctor watch you closely. The baby may grow too slowly or develop other complications.

• Types of Miscarriage
• Minimizing the Risk of Miscarriage  
• After Miscarriage