What Causes a Miscarriage?


I was utterly terrified of having a miscarriage during my first pregnancy. I was also pretty uninformed. The only statistic I could focus on was that 10-20% of pregnancies end in a miscarriage. I cut out all caffeine, even abstaining from chocolate and green tea. I stopped exercising, except for a few short walks once in a while. I literally worried myself sick over it (although that could have been morning sickness, too).

All’s well that ends well, though, and everything was fine. But it still seems like every other woman I meet has a story about how she miscarried or how someone she knows had a miscarriage. (By the way, if you’re talking to a pregnant woman, don’t bring up miscarriages unless she does. It really puts a damper on the conversation.)

This time around, I wanted to get my facts straight before I panicked for no reason. So, here are the answers to all your questions about miscarriages.

What is a miscarriage?
A miscarriage is the spontaneous loss of a pregnancy before the 20th week of pregnancy.

What are the risks of miscarrying?
I’ve seen statistics that range from 10-25%. You might hear some statistics that claim the rate of miscarriage is even higher. The March of Dimes, for example, states that as many as half of pregnancies may end in miscarriage. That’s because they’re including chemical pregnancies, which usually happen before a woman even knows she is pregnant.

More than 80% of miscarriages occur within the first three months of pregnancy. According to the book Your Pregnancy Week by Week, these are the most important dates:

  • 6 Weeks – At this point, an ultrasound can usually detect the yolk sac. Risk of miscarriage is about 15%.
  • 8 Weeks – If you can see the fetal heartbeat on a scan, your risk of miscarriage has fallen to just 3%. If the heartbeat is very slow, though, you may have a higher risk of miscarriage. (The actual risk varies depending on how far along you are in the pregnancy; see this study for specifics).
  • 12 Weeks – From here on out, the risk of miscarriage is no more than 1%.

What factors increase my odds of miscarriage?

Keep in mind that these statistics are for the total population. You, personally, may be more (or less) at risk of having a miscarriage. These factors may put you at a higher risk for having a miscarriage:

  • Race – African American women have a higher risk of miscarriage than white women, particularly in weeks 10 through 20.
  • Age – The older you are, the more likely you are to have a miscarriage. A study on over half a million women in Denmark showed the following odds of miscarriage (excluding ectopic pregnancies, stillbirths, and induced abortions)
Maternal Age Odds of Miscarriage
12-19 10.8%
20-24 9.1%
25-29 9.7%
30-34 12.4%
35-39 20.7%
40-44 43.7%
45+ 81.3%
  • Paternal Age – The age of the father can also impact the odds of miscarriage. One study showed that risk of miscarriage is 27% higher in women with a male partner who was 35 or older. Another found that the miscarriage risk increased 60% if the father was 40 or older. Note that this doesn’t mean that if your significant other is over 40, you have a 60% chance of miscarriage. It just means that your odds of miscarrying (which may be fairly low already) are 60% higher.
  • Previous MiscarriagesA single miscarriage does not significantly increase your odds of miscarrying again. But the more consecutive miscarriages you have, the more your risk increases. After two consecutive miscarriages, you have a 25% chance of miscarrying a third time. After three consecutive miscarriages, there is a 35% chance that the next pregnancy will also end in a miscarriage. If you’ve incurred three or more consecutive miscarriages, you’ll want to see your health care provider to find out what might be causing them.

How can I decrease my risk of miscarriage?
While you can’t do anything about your age, race, or past miscarriages, there are some personal factors that increase your risk of miscarriage. Luckily, you can make changes to your lifestyle before you get pregnant! Here are the most notable factors that increase your risk of miscarriage:

  • Smoking – Women who smoke during pregnancy are more likely to have a miscarriage. If you’re a smoker, quit before you get pregnant. If you wait to quit until you see that positive pregnancy test, you’ve already increased your odds of miscarriage. And if your husband smokes, get him to quit too, since secondhand smoke is also linked to miscarriage.
  • Drinking – Drinking alcohol can also increase the likelihood that you will miscarry. In the United States, the CDC recommends that women who are pregnant or are trying to become pregnant abstain from alcohol entirely. England’s Royal College of Obstetricians and Gynaecologists recently updated their advice to recommend that women in the first trimester avoid alcohol altogether, although they don’t believe that a glass of wine once or twice a week in later trimesters is harmful.
  • WeightIf you are overweight or obese, there’s no better reason to lose the pounds. Women who are obese or overweight (with a BMI of 25.0 or greater) are 67% more likely to have a miscarriage. (Calculate your BMI here.) In many cases of miscarriage in obese women, there were no signs of fetal chromosomal abnormality, which is the primary cause of most miscarriages. Being too thin can also increase your odds of miscarriage; one study found that women with a BMI of less than 20 were also more prone to miscarry.

Nutrition in the Childbearing Years

What causes a miscarriage?
In the vast majority of cases, the reason for a miscarriage isn’t because of something the mother did or didn’t do. It can be very difficult to pinpoint the exact cause of a miscarriage, but these are some of the most common:

  • Chromosomal Abnormalities – For whatever reason, there was something wrong with the way the chromosomes lined up at conception. An estimated 50-66% of miscarriages are associated with chromosomal problems, and it’s unlikely to be a recurring problem.
  • Uterine AbnormalitiesIf your uterus is shaped abnormally or you have a uterine septum, the fetus may have trouble getting nourishment. This can be fixed with surgery.
  • Weak Cervix As the fetus grows, it puts pressure on the cervical opening. If the cervix muscle is weak, it may be unable to stay closed. This is rare, and is more likely to be the cause of miscarriages in the second trimester or later.

What’s the bottom line?
A miscarriage is devastating, but you can’t blame yourself. The vast majority of women who miscarry will go on to have a successful pregnancy – even if they have had recurrent miscarriages. Take good care of yourself while trying to conceive and while you are pregnant, and you’ll know you’ve done everything you can to be successful.

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