Miscarriage - Risk Factors

Risk Factors

Pregnancies involving more than one fetus are considered at increased risk.

Uncontrolled diabetes greatly increases the risk also. Because diabetes may develop during pregnancy (gestational diabetes), an important part of prenatal care is to monitor for signs of the disease.

Polycystic ovary syndrome is a risk factor, with 30–50% of pregnancies in women with PCOS being aborted during the first trimester. Two studies have shown treatment with the drug metformin significantly lowers the rate of abortion in women with PCOS (the metformin-treated groups experienced approximately one-third the miscarriage rates of the control groups). A 2006 review of metformin treatment in pregnancy found insufficient evidence of safety, however, and did not recommend routine treatment with the drug.

High blood pressure during pregnancy, known as preeclampsia, is sometimes caused by an inappropriate immune reaction (paternal tolerance) to the developing fetus, and is associated with the risk of spontaneous abortion. Similarly, women with a history of recurrent spontaneous abortions are at risk of developing preeclampsia.

Severe cases of hypothyroidism increase the risk of miscarriage. The effect of milder cases of hypothyroidism on miscarriage rates has not been established. The presence of certain immune conditions such as autoimmune diseases is associated with a greatly increased risk. The presence of anti-thyroid autoantibodies is associated with an increased risk with an odds ratio of 3.73 and 95% confidence interval 1.8–7.6.

Certain illnesses (such as rubella and chlamydia) increase the risk.

Tobacco (cigarette) smokers have an increased risk of spontaneous abortion. An increase in the rates also is associated with the father being a cigarette smoker. The husband study observed a 4% increased risk for husbands who smoke fewer than 20 cigarettes/day, and an 81% increased risk for husbands who smoke 20 or more cigarettes/day.

Cocaine use increases the rates. Physical trauma, exposure to environmental toxins, and use of an IUD during the time of conception have also been linked to increased risk.

Antidepressants especially paroxetine and venlafaxine can lead to spontaneous abortion.

The age of the mother is a significant risk factor. Spontaneous abortion rates increase steadily with age, with more substantial increases after age 35.

Several other factors have been correlated with higher rates in some research, but whether they cause the abortions is debated. No causal mechanism may be known, the studies showing a correlation may have been retrospective (beginning the study after the abortions occurred, which may introduce bias) rather than prospective (beginning the study before the women became pregnant), or both. A greater correlation has been shown in the following categories, however.