Risk factorsMultiple pregnancyPregnancies of more than one fetus, i.e. twins, triplets, etc., are considered at increased risk. The more fetuses in the womb, the higher the risk.
Intercurrent diseasesSeveral intercurrent diseases in pregnancy can potentially increase the risk of miscarriage, including:
- Diabetes mellitus; The risk of miscarriage is increased in women with poorly controlled insulin-dependent diabetes mellitus. This 1998 prospective study found that the risk increased by 3.1% (over the background risk of about 16%) for each standard deviation in glycosylated haemoglobin above the normal range. The risk was not found to be significantly increased in women with good glycaemic control in early pregnancy.
- Polycystic ovary syndrome, may increase the risk of miscarriage, but this is disputed. Two studies suggested treatment with the drug metformin significantly lowers the rate of miscarriage in women with PCOS, but the quality of these studies has been questioned. A 2006 review of metformin treatment in pregnancy found insufficient evidence of safety, and did not recommend routine treatment with the drug. In 2007 the Royal College of Obstetricians and Gynecologists also recommended against use of the drug to prevent miscarriage.
- Hypothyroidism; 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.
- Vertically transmitted infections; Certain vertically transmitted infections (such as rubella and chlamydia) increase the risk.
- Autoimmune disease; Some research suggests autoimmunity as a possible cause of recurrent or late-term miscarriages. Autoimmune disease occurs when the body's own immune system acts against itself. Therefore, in the case of an autoimmune-induced miscarriages the woman's body attacks the growing fetus or prevents normal pregnancy progression. Further research also has suggested that autoimmune disease may cause genetic abnormalities in embryos which in turn may lead to miscarriage. As an example, Celiac disease increases the risk of miscarriage by an odds ratio of approximately 1.4.
- Mycoplasma genitalium infection is associated with increased risk of preterm birth, and spontaneous abortion.
See also: Smoking and pregnancyTobacco (cigarette) smokers have an increased risk of miscarriage. 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.
Further information: Advanced maternal ageThe age of the mother is a significant risk factor. Miscarriage 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 miscarriages is debated. No causal mechanism may be known, the studies showing a correlation may have been retrospective (beginning the study after the miscarriages occurred, which may introduce bias) rather than prospective (beginning the study before the women became pregnant), or both.