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.