Distinguishing Miscarriage Risk Factors from Miscarriage Causes

Risk Factors for Miscarriages
The research is sometimes confusing on which factors cause miscarriage vs. have a statistical association with miscarriage.. Image © Craig Smallish / Getty Images

It can be very confusing to read anything about the reasons for miscarriage, given that there's so much conflicting information out there. Some ideas are reported as facts in one place and myths in another. For example, you may have read in one place that "stress causes miscarriages," but in another place you might read, "there's no evidence that stress can cause miscarriages." What gives?

The answer often lies in a confusion between miscarriage causes and miscarriage risk factors.

The two are not identical, and understanding the distinction is important when you try to interpret information on the Internet. If you have ever heard the old saying that "correlation is not causation," you are familiar with this phenomenon. Simply because two factors are associated statistically does not mean that one caused the other.

Example of Difference Between Correlation and Causation

To understand why correlation does not equal causation in terms of miscarriage causes, let's look at an example. Studies have shown that women who have tooth problems (periodontal disease) are more likely to give birth prematurely, have babies with a low birth weight, and possibly even more likely to miscarry.

But does that mean that tooth problems cause pregnancy complications? It's possible. Tooth decay bacteria could plausibly secrete some kind of yet undiscovered substance that trigger preterm labor.

But there are other equally plausible reasons why there might be a link. For example, smoking increases risk of both tooth decay and premature birth, as does having diabetes. Women with tooth decay might lack adequate dental coverage, which could mean they have inadequate health care coverage, and thus that they do not get care that might otherwise prevent premature birth.

With these alternate explanations, tooth decay could simply mean a woman has increased risk for premature birth and not that her tooth decay was what caused her baby's premature birth. Thus, tooth decay would be a risk factor for premature birth but not necessarily a cause of premature birth.

Correlation and Causation in Miscarriage Research

This same mechanism applies to quite a few theories regarding miscarriage causes. There are loads of factors that have been linked to risk of miscarriage in research, and researchers have theorized that many of those factors might indeed cause miscarriages, but very few of those theories are proven.

Going back to the example of stress, research has shown that having high stress is associated with increased risk of miscarriage, but no one knows at this point whether stress actually causes miscarriage due to something like hormonal fluctuations in the body or whether there's another explanation. For example, stress could mean that women are more likely to drink alcohol or engage in other behaviors that independently increase the risk of miscarriage.

Another example is the idea that low progesterone causes miscarriages. Women who miscarry are likely to have low progesterone, but there is a lot of controversy over whether the low progesterone is the actual cause of the miscarriage or whether it's merely a sign that a pregnancy is destined to miscarry.

Some other theorized factors have been correlated to miscarriage risk in the past but have not been proven to cause miscarriages. Some of them might actually cause miscarriages, but many of them may not.

What to Do About Miscarriage Risk Factors

So what does this mean for someone who wants the lowest possible risk of miscarriage? Well, quite a few risk factors for miscarriage are things that aren't good for your health anyway and you cannot go wrong with trying to address them. For example, if you smoke, quitting smoking is never a bad idea. It makes perfect sense to try to address these factors in your life in order to improve your overall health. A lower risk of miscarriage could be a side benefit.

At the same time, you should not look back in time and try to blame a specific miscarriage on any risk factor you might happen to have. Unless doctors did testing on the tissue from your miscarriage, you can't know what caused the miscarriage -- and even with testing, you still might not know. Even if you ate a lot of trans fats during your pregnancy, for example, that doesn't mean you can assume the trans fats caused your miscarriage.

It's quite likely that the trans fats had no effect whatsoever. But it's certainly a good idea to address your trans fat intake in the future for numerous reasons that have nothing to do with miscarriage.

In addition, if you're having recurrent miscarriages, you may choose to work with a doctor who promotes an unproven therapy against a risk factor that could theoretically be a miscarriage cause.

This is an understandable choice, although it is wise to always know which therapies are proven vs. not proven so as to keep expectations grounded.


Krabbendam, Lydia, Luc Smits, Rob de Bie, Janneke Bastiaanssen, Foekje Stelma, and Jim van Os, "The impact of maternal stress on pregnancy outcome in a well-educated Caucasian population." Paediatric & Perinatal Epidemiology Nov 2005. Accessed 20 Oct 2008.

Xiong, X., P. Buekens, W.D. Fraser, J. Beck, and S. Offenbacher, "Periodontal Disease and Adverse Pregnancy Outcomes: A Systematic Review." Obstetrical & Gynecological Survey May 2006. Accessed 20 Oct 2008.

Continue Reading