Odds of Miscarriage with Specific Risk Factors

Breakdown of Pregnancy Loss Odds by Groups

You've likely heard that the "average" risk of miscarriage is roughly 15 to 20 percent. 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.

Risk Factors for Miscarriage - Unavoidable and Avoidable

Some risk factors for miscarriage 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.

Miscarriage and Emotions

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 for miscarriage following a miscarriage, you may want to take a look at these numbers in the company of someone who can be shoulder to cry on. One thing that is worse than miscarrying, is miscarrying without a friend to lean on.

Calculating the Risk of Miscarriage. - Percentages and Odds Ratios

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

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 (note: there are many more risk factors than are listed here) include:

  • 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 percent of miscarriages. (Learn more about pregnancy loss associated with smoking and secondhand smoke.)
  • Secondhand smoke - Exposure to secondhand smoke increases the risk of miscarriage by 11 percent.
  • Weight. For women who are underweight (have a body mass index (BMI) less than 18.5) the odds ration is 1.08. The odds ratio of miscarriage is 1.09 for overweight women and between 1.15 and 1.27 for obese women. (Learn more about obesity and miscarriage.)
  • Prior miscarriage - In contrast to information in the past, getting pregnant within six months of a preceding miscarriage does not increase the risk of a second miscarriage.
  • Preventable infections are thought to account for around 15 percent of miscarriages. Many infections have not been shown to increase risk, or some studies suggest they do and others that they don't. Infections which have been clearly associated with miscarriage include malaria, brucellosis, cytomegalovirus, HIV, dengue fever, influenza virus, and vaginal infection with bacterial vaginosis. (Learn more about bacterial vaginosis and miscarriage.)
  • Maternal age - The risk of miscarriage clearly increases with age, the the risk of having a miscarriage being roughly 50 percent for a woman in her early 40's.
  • Paternal age - While we often think most about maternal age, increased paternal age may also increase the odds of miscarriage.
  • Previous miscarriages - Roughly two percent of women will have two miscarriages in a row, and one percent 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 percent.
  • The risk of miscarriage related to an amniocentesis is now thought to be as low as one in 700.

Table of Miscarriage Rates Associated with 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 FactorPercent Who Will Miscarry
Mother's age 30-3925 percent
Mother's age 40-44~50 percent
Mother's age >45~95 percent
2 prior miscarriages20 percent
3 prior miscarriages40 percent
4 or more prior miscarriages54 percent
-Odds Ratio of Miscarriage
Obesity (BMI > 30)1.15 to 1.27

Being underweight (BMI < 18.5)

Father's age >401.6
Drinking 5 or more units/alcohol per week4.84
Smoking >10 cigarettes daily1.32

Bottom Line on Your Odds of Miscarriage

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 they 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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