When Do Most Miscarriages Occur?

The First 12 Weeks Have the Highest Pregnancy Loss Rate

pregnant woman gets ultrasound
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Many women wonder when most miscarriages occur. At what point in pregnancy will you be able to breathe a sigh of relief and stop worrying so much about having a miscarriage?

Pregnancy is obviously a time of anxiety as well as anticipation, and a lot of pregnant women worry - especially those who have had a miscarriage before. Some seek comfort in statistics, feeling a greater sense of relief after passing specific milestones in pregnancy at which statistical odds of miscarriage decrease.

When Do Most Miscarriages Occur?

Most estimates state that 80 percent of miscarriages happen before 12 weeks of pregnancy. As pregnancy progresses, miscarriage risk decreases. Risk of miscarriage drops significantly after a doctor detects a heartbeat on ultrasound.

A miscarriage, also known as a spontaneous abortion, is a pregnancy loss before the 20th week of gestation. Pregnancy losses after 20 weeks are not classified as miscarriages. They are either considered a stillbirth, where the fetus dies in the uterus, or a neonatal death, where the baby is born alive, but passes away within the first month of life. Risk for both stillbirth and neonatal death is about 1 in 200.

How Common Are Miscarriages?

If you're wondering when miscarriages occur, you may also have questions about how common miscarriages are.

If you search for information on miscarriage statistics, you will find a wide range. Different sources claim that anywhere from 10 to 75 percent of conceptions end in miscarriage.

The top of that range sounds scarier than it is. It's speculated that 75 percent of fertilized eggs may pass through the woman's body without implanting.

By the time you actually get a positive pregnancy test, your risk of miscarriage will be at the lower end of that range. In a 1988 study, researchers followed 221 women over a combined total of 707 menstrual cycles, with 198 total pregnancies.

They found that 22 percent of the pregnancies ended before they could be detected clinically (such as by standard urine pregnancy tests). Including the early miscarriages, the total miscarriage rate was 31 percent. 

However, anywhere between 8 and 20 percent of recognized pregnancies will end in miscarriage.

What Causes Miscarriage?

​About half of miscarriages are caused by chromosomal abnormalities, making these problems the single most common miscarriage cause. Most of these are random one-time occurrences that are not inherited from the parents. 

The earlier in pregnancy a miscarriage occurs, the more likely it was caused by a chromosomal abnormality. 

Usually, these abnormalities are aneuploidies, meaning there is an incorrect number of chromosomes. Trisomy 16 is the most common chromosomal abnormality that causes miscarriage. It occurs when there is an extra copy of chromosome 16, which is not compatible with life. 

The biggest miscarriage risk factor is the mother's age. One study of more than 1 million women who were admitted to a hospital for adverse pregnancy outcomes found an increasing miscarriage risk by maternal age.

While the overall miscarriage rate was 11 percent, miscarriages were more frequent in older women: 

  • age 20 to 30 years: 9 to 17 percent miscarriage rate
  • age 35 years: 20 percent miscarriage rate
  • age 40 years: 40 percent miscarriage rate
  • age 45 years: 80 percent miscarriage rate


Spontaneous abortion: Risk factors, etiology, clinical manifestations, and diagnostic evaluation. UpToDate. February 23, 2015.

Patient information: Miscarriage (Beyond the Basics). UpToDate. July 16, 2015.

Health, United States 2005. National Center for Health Statistics.

Recurrent Miscarriage: Causes, Evaluation, and Treatment. Medscape General Medicine. 1998;1(3).

Wilcox, A.J., Weinberg, C.R., O'Connor, J.F., et al. (1988). Incidence of Early Loss of Pregnancy. New England Journal of Medicine.

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