Risk factors for miscarriage. Natural abortion.
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:
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.