What Causes Miscarriage to Happen?

  • 90%

    of losses occur due to chromosomal abnormalities based on statistics. However, testing isn't always available to determine the cause.

  • It's common to wonder why pregnancy loss happened and if you could have done anything to prevent it — but the truth is you had no ability to change the outcome. Pregnancy loss is in no way your fault.

  • % Risk of Pregnancy Loss by Age

    The age you become pregnant is the most significant risk factor because of the link between age and chromosomal abnormalities.

  • Share your experience on pregnancy loss to be a part of making change

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If you are experiencing a miscarriage, you might be asking "why did this happen?". Pregnancy loss is difficult, confusing, and often grief-inducing. The hardest part is that, quite often, it’s hard to know exactly why a loss occurs. While statistics show that chromosomal abnormalities are typically the cause of pregnancy loss (up to 90% of the time), testing to determine why this happened isn’t always available, which can leave you feeling in the dark about your experience. What’s important to remember is that 98% of people who experience pregnancy loss go on to become pregnant again and have full-term, healthy pregnancies and babies.

Why do miscarriages happen?

When experiencing pregnancy loss, people often wonder why it happened and if there was anything they could have done to prevent it. This is natural, but the truth is that most pregnancy losses are due to chromosomal abnormalities — meaning that when the sperm and egg united, the appropriate genetic material was not present and never would have grown into a healthy pregnancy from the beginning. This is an incredibly complex process, and sometimes it doesn’t go perfectly. The body gets it right on average 75% of the time (25% of pregnancies end in loss), and you had no ability to change the outcome for better or for worse. Pregnancy loss is in no way your fault. 

Chromosomal abnormalities

Of the pregnancy losses that are tested, at least half, but possibly as much as 90%, of pregnancy losses before 20 weeks are the result of random chromosomal abnormalities. In many cases, the new embryo will have more or fewer than the 23 chromosome pairs it’s meant to have.

Undetermined cause

Determining the cause of miscarriage can be tricky. While testing is available, results can be inconclusive or unclear and the majority of recurrent miscarriages do not have a source that can be determined. And in some instances, testing may not be available at all. While not having an answer can be difficult to sit with, remember that conception and pregnancy involve incredibly complex processes, and the body doesn’t get them all right every single time. Pregnancy loss is not an indication that your body does not have the ability to work properly.

What are risk factors for pregnancy loss?

There are some factors that put you at higher risk for pregnancy loss, including:


The age you become pregnant brings the most significant risk factor for pregnancy loss because of the association between age and chromosomal abnormalities, the leading cause of pregnancy loss. According to a national prospective cohort study of over 421,000 pregnancies, the risk of early pregnancy loss for different age groups looks like:

  • under 20 years old: 17%
  • 20 to 24 years old: 11%
  • 24 to 29 years old: 10%
  • 30 to 34 years old: 11%
  • 35 to 39 years old: 17%
  • 40 to 44 years old: 33%

    Previous pregnancy losses

    About 98% of people go on to have a healthy pregnancy the next time they become pregnant. If you do experience two consecutive losses, that doesn’t mean you’ll never get pregnant. In fact, the predicted risk of pregnancy loss in a future pregnancy only goes up from 20% to 28% after two consecutive pregnancy losses. After three or more consecutive losses, the risk of another pregnancy loss is about 43%.

    Anatomical factors

    Factors like uterine fibroids, polyps, and adhesions have been associated with pregnancy loss, but their influence depends on the size and exact location, and many people (as much as 70 to 90%) have fibroids that do not affect pregnancy at all. In a study of 104 people with recurrent pregnancy loss of multiple types, the most common structural diagnoses that likely contributed to the loss were intrauterine adhesions (15%), fibroids (14%), uterine septum (3%), and endometrial polyps (2%). These might not be identified prior to a pregnancy loss, but once known, they can typically be removed before the next pregnancy.

    Health conditions

    Certain health conditions are associated with an increased risk for pregnancy loss, including:


    While you can have a healthy pregnancy if you are overweight or obese, research shows that having a BMI of greater than 25 is associated with an increased risk of early pregnancy loss. For example, research carried out on genetic analyses of 204 fetuses lost by those with an average age of 35 showed that of the 153 people with a BMI of less than 25, 36.6% had lost fetuses compared with 52.9% of the 51 with a BMI over 25. For example, research examining 204 instances of pregnancy loss among people with an average age of 35 found that chromosomal abnormalities accounted for about 59% of these losses, but that being overweight or obese played a significant role in the loss of fetuses with healthy chromosomes. To break down the effect of weight further: About 37% of the 153 people with a BMI of less than 25 lost their pregnancy, while about 53% of the 51 people with a BMI over 25 lost their pregnancy.


    The role between PCOS (polycystic ovary syndrome) and pregnancy loss is constantly under scrutiny and the risk varies a lot depending on the study. What we know is that a high BMI, high inflammation, and high insulin levels — which are often associated with PCOS — can increase pregnancy loss rates. That said, people with PCOS who don’t have these additional conditions do not necessarily have a higher risk for pregnancy loss.


    Endometriosis, a condition where endometrial tissue is found outside the uterus, is often associated with pregnancy loss. But the condition is often undiagnosed and probably more common than we know, thus it is likely that millions of people with endometriosis have babies every year. Our knowledge of endometriosis is constantly growing, but there is evidence that it can affect fertility by causing scarring of the pelvis, specifically of the fallopian tubes, making it challenging for the egg and sperm to meet and fertilize. It can also play a role in antagonizing progesterone levels and causing various levels of inflammation, which may be more likely to contribute to higher pregnancy loss rates.

    Thyroid disease

    Both hyper- and hypothyroidism have been associated with an increased risk of pregnancy loss when thyroid levels are significantly outside the norm. If you have a known thyroid issue or suspect one, it’s a good idea to see your doctor prior to trying to conceive to have your levels checked as thyroid production needs to rise during pregnancy.

    Untreated infections

    About 15% of early pregnancy losses are associated with an infection, such as untreated syphilis, maternal cytomegalovirus, or parvovirus B19 infection.


    Paternal health factors

    It’s not often the immediate focus, but the health of both parents can influence the ability for a pregnancy to continue to term.

    Sperm quality

    Researchers have linked repeat pregnancy losses to sperm quality. For example, one study investigated the sperm quality of 50 men whose partners had suffered three or more consecutive losses and found that compared to men whose partners had not experienced pregnancy losses, the sperm of those involved in the study had higher levels of DNA damage.

    Metabolic disease

    Studies show that compared to men with no metabolic syndrome, the odds of pregnancy loss increased by 10% for those with one metabolic risk component of metabolic syndrome. That risk rose to 19% for men with three metabolic syndrome components.

    Significant stressors and trauma

    Everyday stressors such as a busy time at work, a move, or caring for a sick loved one are unlikely to contribute to pregnancy loss. That said, acute and chronic stressors that affect your safety, security, or access to appropriate medical care — such as violence, food or housing insecurity, and financial or racial disparities — may increase the risk of pregnancy loss.


    There may be some genetic component that affects pregnancy loss risk. At least one study has shown that people who experience repeat pregnancy loss have an 11% increased odds of having a mother with a history of pregnancy loss.

    Getting pregnant while an IUD is intact

    IUDs are extremely effective, but failures do happen. If you get pregnant while you have an IUD, evidence suggests that having it removed decreases the risk of pregnancy loss more than leaving it in.

    Medication and substance use

    It’s difficult to discuss these scenarios because it varies greatly depending on the agent, dose, and timing of exposure. If you want more information, you can talk to your doctor or see the US National Library of Medicine LactMed toxicology data network site. It’s important to talk to your doctor about all your medications, supplements, alcohol, and drug intake before you get pregnant or when you find out you are pregnant.

    Is there anything I can do to prevent miscarriage?

    There’s no shortage of internet advice about how to prevent pregnancy loss and the quality of that advice varies greatly. Here are some common pieces of advice you’ll come across and whether or not they really have evidence to support them.

    Myth: Prenatal vitamins

    A meta-analysis of over 40 studies concluded that use of prenatal vitamins in the preconception, periconception, and early pregnancy time frames does not prevent early pregnancy loss.

    Evidence-based: Folic acid may prevent pregnancy loss

    Taking 400 mcg of folic acid prior to pregnancy can prevent neural tube defects, and it’s also possible that folic acid supplementation can reduce the risk of pregnancy loss, though the research is not entirely conclusive.

    Myth: Cut out caffeine

    There is no evidence that consuming caffeine in moderation (less than 200 mg a day) increases your risk of pregnancy loss. That said, there is some weak evidence that more than 200 mg a day of caffeine — which is the equivalent of more than four espresso shots or more than two standard cups of coffee per day — in the first trimester can increase your risk of pregnancy loss. If you have questions or concerns about caffeine and pregnancy, talk to your medical provider.

    Evidence-based: Vitamin D3 supplements

    In recent years, a possible association between vitamin D3 deficiency and pregnancy loss has been uncovered. Among people who became pregnant, a National Institutes of Health (NIH) study showed that each 10 nanogram per milliliter increase in preconception vitamin D was associated with a 12% lower risk of pregnancy loss. The same NIH study showed that those who had sufficient vitamin D concentrations before conception were 10% more likely to become pregnant and 15% more likely to have a live birth, compared to those with insufficient concentrations of the vitamin. Because there is also data that shows low vitamin D levels can increase your risk for various cancers, weaken your immune system generally, and may play a role in preeclampsia and preterm labor, it is wise to replenish vitamin D levels before you become pregnant.

    Myth: Progesterone supplementations

    Though popular, progesterone supplements have not been associated with a decreased risk of pregnancy loss. Defer to your medical provider when deciding if progesterone therapy is an option for you.

    Evidence-based: Maintain a healthy weight

    Your risk for pregnancy loss increases when you are both underweight or overweight.

    Evidence-based: Stop smoking and avoid secondhand smoke

    There is a well-established link between pregnancy loss risk and tobacco use. One large meta-analysis found that any active smoking during pregnancy increased the risk of pregnancy loss. The data showed that the relative risk increased with increasing numbers of cigarettes smoked per day, which means that even cutting back can make a difference. For 1 to 10, 11 to 19, and 20 or more cigarettes per day, the risk increased by 8%, 25%, and 42%, respectively. The same study showed that second hand smoke exposure during pregnancy increased the risk of pregnancy loss by 11%.

    Evidence-based: Reduce bisphenol A (BPA) exposure

    BPA is an estrogenic compound found in many household items that has been found in higher levels in follicular and endometrial fluid in those who lost a pregnancy. The best way to reduce your exposure to BPA is to avoid microwaving plastic food containers, avoid plastics with recycle codes 3 or 7, reduce consumption of canned foods that are not labeled “BPA free,” and whenever possible opt for glass, porcelain, or steel containers over plastic.

    Explore our hub on pregnancy loss here.

    Bodily does not provide medical advice, diagnosis, or treatment. The resources on our website are provided for informational purposes only. You should always consult with a healthcare professional regarding any medical diagnoses or treatment options.
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