What Causes Miscarriage to Happen?

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.

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.

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:

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%.

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.

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

Obesity

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.

PCOS

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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%.

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.