Calculating the Odds of Miscarriage

miscarriage explanation
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You've likely heard that the "average" risk of miscarriage is roughly 15 to 20%. Yet, since there are several factors which may increase risk, how can you estimate your personal risk of having a miscarriage? Let's take a look at specific risk factors for miscarriage, and how much risk these confer.

Some risk factors are unavoidable. You can't change the year you were born or whether you have had miscarriages in the past. You can, however, look at avoidable risk factors, such as alcohol intake, in order to reduce your risk.

Emotions Associated With Miscarriage

Before delving into the cold numbers regarding miscarriage, it's important to take a moment and mention the emotions of miscarriage. For many women, simply hearing the word "miscarriage" may give rise to tears. No matter how common miscarriage is, if it happens to you it can be devastating. The platitudes people often hear, such as, "you can always get pregnant again" do nothing to take away the hurt.

If you are trying to learn what your risk factors are following a miscarriage, you may want to take a look at these numbers in the company of someone who can be a shoulder to cry on. One thing that is worse than miscarrying, is miscarrying without anyone to lean on.

Odds Ratios of Miscarriage Risk

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, an 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. To determine the likelihood of miscarriage, you can multiply the number of people who ordinarily are expected to miscarry by this number.

For example, an odds ratio of 1.5 means that a person is 50% more likely than the "average" woman to miscarry. This does not mean a 50% chance of a miscarriage occurring. For example, if the risk of miscarriage without the risk factor is 20%, someone with that risk factor would have a 30% risk of miscarrying (because 30% is 50% more than 20%, or 20% multiplied by 1.5 is 30%.)

Known Risk Factors for Miscarriage

There are several risk factors for miscarriage, including the age of the mother and father, lifestyle factors, and medical conditions. These risks—either the percent who will miscarry or the odds ratio of miscarriage are listed in the table below.

Some of the risk factors which have been evaluated statistically include:

  • Amniocentesis: The risk of miscarriage related to amniocentesis is now thought to be as low as one in 700.
  • Maternal age: The risk of miscarriage is roughly 50% for a woman in her early 40s.
  • Paternal age: While we often think most about maternal age, increased paternal age may also increase the odds of miscarriage.
  • Preventable infections: This is thought to account for around 15% of miscarriages. Many infections have not been shown to increase risk, or some studies suggest they do and others that they don't. Infections that have been clearly associated with miscarriage include malaria, brucellosis, cytomegalovirus, HIV, dengue fever, influenza virus, and vaginal infection with bacterial vaginosis. 
  • Previous miscarriages: Roughly 2% of women will have two miscarriages in a row, and 1% of women, three or more. While the odds ratio for miscarriage goes up for each subsequent miscarriage, it is noteworthy that even with four or more prior miscarriages, the percent of women who will not miscarry is close to 50%.
  • Prior miscarriage: In contrast to information in the past, getting pregnant within 6 months of a preceding miscarriage does not increase the risk of a second miscarriage.
  • Secondhand smoke: Exposure to secondhand smoke increases the risk of miscarriage by 11%.
  • Smoking: The risk related to "any smoking" in pregnancy carries an odds ratio of 1.23, which increased to 1.32 when evaluating the pregnancy in which miscarriage occurred. It's thought that smoking accounts for up to 10% of miscarriages. 
  • Weight: For women who are underweight (have a body mass index (BMI) less than 18.5) the odds ratio is 1.08. The odds ratio of miscarriage is 1.09 for overweight women and between 1.15 and 1.27 for obese women.

Note: There are several other risk factors beyond those listed here.

Miscarriage Rates and Risk Factors

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.

Miscarriage Rates Associated With Risk Factors

Risk Factor Percent Who Will Miscarry
Mother's age 30-39 25 percent
Mother's age 40-44 ~50 percent
Mother's age >45 ~95 percent
2 prior miscarriages 20 percent
3 prior miscarriages 40 percent
4 or more prior miscarriages 54 percent
- Odds Ratio of Miscarriage
Obesity (BMI > 30) 1.15 to 1.27

Being underweight (BMI < 18.5)

1.08
Father's age >40 1.6
Drinking 5 or more units/alcohol per week 4.84
Smoking >10 cigarettes daily 1.32

A Word From Verywell 

As noted above, there are some preventable risk factors for miscarriage that women can work to control, but most of the time there is nothing you can do to prevent a miscarriage, and having a miscarriage does not mean that you did anything wrong.

They just happen. Sometimes miscarriages occur due to a chromosomal abnormality in the baby, yet hearing this does not help when you miscarry your baby. It still hurts as much.

You are not a statistic. If you've had a miscarriage or are concerned you may have a miscarriage, talk to your doctor and reach out for support from your loved ones. Nobody should have to go through the anxiety related to miscarriage alone.

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  1. Kleinhaus K, Perrin M, Friedlander Y, et al. Paternal Age and Spontaneous Abortion. Obstet Gynecol. 2016;108(2):369-77. doi:10.1097/01.aog.0000224606.26514.3a

  2. Giakoumelou S, Wheelhouse N, Cushieri K, et al. The Role of Infection in Miscarriage. Hum Reprod Update. 2016. 22(1):116-133. doi:10.1093/humupd/dmv041

  3. Garrido-Giminez C, Alijotas-Reig J. Recurrent miscarriage: causes, evaluation and management. 2014;91(1073). doi:10.1136/postgradmedj-2014-132672

  4. Pineles B, Park E, Samet J. Systematic Review and Meta-Analysis of Miscarriage and Maternal Exposure to Tobacco Smoke During Pregnancy. Amer J Epidemiol. 2014;179(7):807-23. doi:10.1093/aje/kwt334

  5. Balsells M, Garcia-Patterson A, Corcoy R. Systematic Review and Meta-Analysis on the Association of Prepregnancy Underweight and Miscarriage. Eur J Obstet Gynecol Reprod Biol. 2016;207:73-79. doi:10.1016/j.ejogrb.2016.10.012

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