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.
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. 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. 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.