Spontaneous abortion, or miscarriage, occurs in about 15 to 20 percent of all pregnancies, regardless of how they were conceived. Most early losses are a result of the conceptus being genetically abnormal. As few as one in three human conceptions is genetically normal, but most of the genetically abnormal conceptions are lost before the woman even knows she is pregnant. Even so, some genetically abnormal conceptions do survive long enough to result in a recognized pregnancy. These are often conceptions that have three sets of chromosomes (triploidy) rather than two, or conceptions that have an extra of just one chromosome (trisomy). These may present as an empty sac, or blighted ovum, which is a pregnancy in which there is no identifiable fetal tissue, or as a fetus that develops only briefly and then ceases. The human genetic message is very specific, and any variation from the normal set of 46 chromosomes is rarely compatible with life.
Spontaneous abortion is, then, one possible outcome any time a conception occurs, and the chances are about one in five that any clinically recognized pregnancy will be aborted. And the chances do increase as the woman gets older. "Older" eggs don't divide as well when they get fertilized, and therefore the resulting pregnancies are more often abnormal. The chances of spontaneous abortion rises to as high as 50 percent in women in their early to mid 40s.
So how do we decide whether the loss of a pregnancy is just chance or the result of some other underlying problem that requires investigation? Most of the time, a single loss is attributed to "bad luck" unless some other factor is apparent. During their reproductive lifespan, most women will experience at least one miscarriage. Should two, or certainly three, losses occur without any successful pregnancies in between, however, evaluation is warranted. There are no hard and fast answers as to whether investigation should be initiated after two or three losses. The chances of another loss do increase slightly after two losses, particularly in older couples. Some investigation may be warranted at this time. After three losses, there is no question.
Causes of Recurrent Pregnancy Loss
The identifiable causes of recurrent pregnancy loss fall into one of seven categories; genetic, infection, uterine anomalies, endocrinologic disorders, cycle abnormalities, autoimmune disorders, and alloimmune disorders.
If the chromosomes of couples who have experienced three consecutive losses are analyzed, an abnormality in the chromosomes of either the man or the woman will be found in one to two percent of couples. There are many types of abnormalities, but the most common are translocations, in which a piece of one chromosome is hooked onto another chromosome (Robertsonian), or in which pieces of chromosomes are exchanged with one another (reciprocal). The implications of a chromosomal abnormality for the chances of a normal conception in the future are totally dependent on the type of abnormality discovered. If an abnormality is uncovered, counseling with a geneticist should be obtained to discuss its implications. Chromosome analyses are performed on blood samples taken from each partner.
While there is no question that infection can result in the loss of a pregnancy, whether or not it can cause recurrent losses is less clear. In couples who have experienced three losses, it is reasonable to treat both partners with a 10-day course of a tetracycline to eliminate infection as a possible cause. Cultures are not necessary.