Calculating the Odds of Miscarriage

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Overall, the "average" risk of miscarriage is estimated to be between 12% and 15% of recognized pregnancies by 20 weeks gestation. Yet, since there are multiple key factors, such as maternal age and drinking, that 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.

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 due to 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" pregnant person 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 known 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 most common risk factors that have been evaluated statistically include:

  • Amniocentesis: The risk of miscarriage related to amniocentesis is now thought to be as low as .35% or about 1 in 300.
  • Drinking Alcohol: Overall, research shows a 1.19 odds ratio of pregnancy loss with exposure to alcohol during pregnancy compared with those that abstained. Drinking more alcohol during pregnancy increases miscarriage risk, with the risk increasing an estimated 6% to 13% with each additional drink consumed per week.
  • Maternal age: The risk of miscarriage is roughly 50% for a woman in her early 40s and up to 75% at age 45.
  • Paternal age: While we often think most about maternal age, increased paternal age also increases the odds of miscarriage, although to a lesser extent than increased maternal age.
  • Preventable infections: Infections are thought to account for around 15% of miscarriages. Many infections have not been shown to increase risk, or studies have shown mixed results. 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, will have 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 those women who will not miscarry is close to 50%.
  • Recent 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. In fact, a shorter interval of fewer than 3 months provides a lower risk of having another miscarriage than occurs with a pregnancy that begins 6 months or more after miscarriage.
  • Secondhand smoke: Exposure to secondhand smoke increases the risk of miscarriage by 11%.
  • Smoking: The risk related to smoking in pregnancy carries an odds ratio of 1.23, which goes up to 1.32 for those smoking 10 or more cigarettes daily. It's thought that smoking accounts for up to 10% of miscarriages. It's important to note that former smokers and smokers that stopped smoking early on in pregnancy are at lower risk, so it's very beneficial to quit smoking before getting pregnant and/or once you find out.
  • Weight: For women who are underweight—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.
  • Working atypical and/or long hours: Compared with those working day shift jobs, the risk of pregnancy loss for night shift workers is 1.21. Working longer hours than is typical also ups the odds ratio for miscarriage to 1.38.

Body Mass Index (BMI) is a dated, biased measure that doesn’t account for several factors, such as body composition, ethnicity, race, gender, and age.

Despite being a flawed measure, BMI is widely used today in the medical community because it is an inexpensive and quick method for analyzing potential health status and outcomes.

Miscarriage Rates and Risk Factors

The table below illustrates overall estimated miscarriage rates by specific risk factors. Note that some of these factors are not yet well understood and that the exact figures vary by individual study. Also, your personal risk may vary significantly based on the specifics of your pregnancy and medical history.

Typical Miscarriage Rates for Risk Factors

Risk Factor Percent Who Will Miscarry
Mother's age 30-39 25%
Mother's age 40-44 50%
Mother's age >45 75%-95%
2 prior miscarriages 20%
3 prior miscarriages 40%
4 or more prior miscarriages 54%
- 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 alcohol, added risk per drink 1.06 to 1.13 (6% to 13% increase in added risk per additional weekly drink)
Smoking >10 cigarettes daily 1.32

A Word From Verywell 

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.

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. While miscarriages most often occur due to a chromosomal abnormality in the baby, knowing this may not help when you miscarry your baby. It still hurts so 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.

12 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Krissi Danielsson
Krissi Danielsson, MD is a doctor of family medicine and an advocate for those who have experienced miscarriage.