Odds of Miscarriage with Specific Risk Factors

Breakdown of Pregnancy Loss Odds by Groups

Various risk factors, both avoidable and unavoidable, can increase an individual woman's risk of miscarriage.

For some risk factors, researchers do not have a flat estimate of the percentage of normal pregnancies that will miscarry. Instead, they have calculated odds ratios to indicate the increased risk compared to women without the risk factor.

An odds ratio shows the percentage of increased risk rather than the risk of miscarriage.

For example, a odds ratio of 1.5 would mean that a woman was 1.5 times more likely to miscarry because she had a particular risk factor (that is, 50% more likely). This would not mean that a person has a 50% chance of miscarriage, but rather that if a person without that risk factor had a 20% risk of miscarriage, someone with that risk factor would have a 30% risk of miscarriage (because 30% is 50% more than 20% -- is your head spinning yet?)

The table below illustrates overall estimated miscarriage rates by specific risk factor. Note that some of these factors are not yet well understood and that the exact figures vary by individual study.


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Miscarriage Rates Associated with Risk Factors

Risk FactorPercent Who Will Miscarry
Mother's age 30-3925%
Mother's age 40-44~50%
Mother's age >45~95%
2 prior miscarriages20%
3 prior miscarriages40%
4 or more prior miscarriages54%
-Odds Ratio of Miscarriage
Being underweight (BMI < 18.5)1.24
Father's age >401.6
Drinking 5 or more units/alcohol per week4.84
Smoking >10 cigarettes daily1.4

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