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.

Sources:

Chatenoud, Liliane, Fabio Parazzini, Elisabetta Di Cintio, Giovanni Zanconato, Guido Benzi, Renata Bortolus, and Carlo La Vecchia, "Paternal and Maternal Smoking Habits before Conception and During the First Trimester Relation to Spontaneous Abortion. Annals of Epidemiology 1998. Accessed 11 Feb 2008.

Everett, Christopher, "Incidence and outcome of bleeding before the 20th week of pregnancy: prospective study from general practice." BMJ 1997. Accessed 8 Feb 2008.

Heffner, Linda J., "Advanced Maternal Age -- How Old Is Too Old?" New England Journal of Medicine Nov 2004. Accessed 8 Feb 2008.

Helgstrand, Stine, and Anne-Marie Nybo Andersen, "Maternal underweight and the risk of spontaneous abortion." Acta Obstetricia et Gynecologica Scandinavica Dec 2005. Accessed 11 Feb 2008.

Kleinhaus, K., M. Perrin, Y. Friedlander, O. Paltiel, D. Malaspina, and S. Harlap, "Paternal Age and Spontaneous Abortion." Obstetrics and Gynecology 2006. Accessed 11 Feb 2008.

Lashen, H., K. Fear, and D.W. Sturdee, "Obesity is associated with increased first trimester and recurrent miscarriage: matched case-control study." Human Reproduction 2004. Accessed 11 Feb 2008.

Parazzini, Fabio, Barbara, Acaia, Orietta Ricciardiello, Luigi Fedele, Paola Liati, and G. Battista Candiani, "Short-term reproductive prognosis when no cause can be found for recurrent miscarriage." BJOG 1988. Accessed 11 Feb 2008.

Rasche, Vibeke, " Cigarette, alcohol, and caffeine consumption: risk factors for spontaneous abortion. Acta Obstetricia et Gynecologica Scandinavica 2003. Accessed 11 Feb 2008.

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
Obesity1.2
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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