Miscarriage Causes: Factors That Increase the Risk

While most pregnancies result in healthy babies, there’s always a risk of miscarriage. It’s an unfortunate circumstance, but it’s not rare, occurring in as many as one in five known pregnancies.

“Miscarriages are much more common than people think,” says Caitlin Dunne, M.D., FRCSC, a reproductive endocrinologist and fertility specialist in Vancouver. “Although it is often emotionally distressing, it is a normal part of reproduction. I think it’s important to talk about miscarriage and normalize the experience to help reduce the stigma, shame, and blame that often comes with miscarriage.”

Here’s what expectant parents and anyone planning to get pregnant need to understand about miscarriage — and how they can lower their risk.

What is miscarriage?

Miscarriage, or pregnancy loss, is defined as the spontaneous loss of an intrauterine pregnancy before 20 weeks gestation. Early pregnancy loss occurs before 13 weeks gestation. It’s estimated that 80% of miscarriages occur before 12 weeks.

Miscarriage is very common, occurring in 10% to 20% of known pregnancies. The term “known” is essential here, as it’s likely more miscarriages happen before people even know they are pregnant.

The term “stillbirth” is used to describe pregnancy loss after 20 weeks and is far less common than miscarriage.

The most common symptoms of miscarriage are bleeding and cramping. Miscarriages typically have symptoms after the gestational sac and embryo start to form, at about 6 to 7 weeks, Dunne explains.

“The mother might have spotting or bleeding that gets heavier, followed by cramps,” Dunne says. “Some people notice when they pass pregnancy tissue, which looks like a greyish-tan-coloured sac. Often, women also pass small to medium-sized blood clots.”

Not always, but decreasing pregnancy symptoms may be a sign of miscarriage, says Fatima Naqvi, M.D., FACOG, an OB-GYN and interim medical director for women’s service at Atlantic Health System in New Jersey. “Some women may know there’s been a change, where they were feeling breast tenderness or nausea, and all of a sudden, it’s starting to feel better,” she says.

But some miscarriages happen and have no symptoms. “A missed miscarriage is when a woman goes for an early pregnancy ultrasound, and the embryo has no heartbeat,” Dunne says. “She may have had no bleeding or cramping whatsoever prior to that diagnosis.”

After symptoms begin or a miscarriage has been diagnosed by ultrasound, the body then must get rid of the pregnancy tissue.

“In cases where the body expels pregnancy tissue on its own without intervention, the cervix will open and the uterus contracts, causing cramping and bleeding until the pregnancy tissue has been expelled,” says Rachel Mandelbaum, M.D., FACOG, a board-certified OB-GYN and reproductive endocrinologist at HRC Fertility in California.

But sometimes, Mandelbaum adds, the body doesn’t expel the tissue on its own. In those cases, the medications mifepristone-misoprostol are administered. Sometimes, a surgical procedure called dilation and curettage (D&C), performed by a physician at a hospital or clinic is needed to complete the process.

What are the common causes of miscarriage?

Miscarriage isn’t someone’s “fault,” but there are some common medical reasons why it happens.

Chromosomal abnormalities

Chromosomal abnormalities are the leading cause of miscarriage, accounting for up to 70%, Naqvi says.

“When I explain to patients what that means, the way I say it is too much genetic material or too little, or something was awry when the egg and the sperm came together,” Naqvi says.

Dunne adds: “Most miscarriages are the result of spontaneous, or natural, genetic mistakes in the embryo. These usually result from the egg and become more common with age, but genetic errors from the sperm are also possible.”

Maternal age

As women age, the quality of their eggs declines. This increases the risk of miscarriage.

“At age 35, the miscarriage rate is about 20%,” says Sara Holt, CNM, MS, a certified nurse midwife at Byenveni Baby in Portland, Oregon. “That’s the same rate for anyone else. But at 40 years old, there’s a miscarriage rate of about 40%. At 45 years old, it’s 75%. It’s a huge difference in incidence, and that’s from egg quality, typically.”

Hormonal imbalances

Hormone fluctuations from hormone-related conditions, such as polycystic ovarian syndrome (PCOS) or thyroid disorders, can make it more difficult to sustain a healthy pregnancy.

Luteal phase defect is another hormonal condition that may contribute to miscarriage, in which the body doesn’t produce enough progesterone to support implantation and pregnancy.

Uterine abnormalities

Structural abnormalities can affect pregnancy, Naqvi says. These include abnormalities in the shape of the uterus, the lining of the uterus, plus anything inside: fibroids, polyps, or even uterine trauma of some kind.

Infections

Some sexually transmitted infections can either cause or increase the risk of miscarriage, as well as specific birth defects. These include untreated gonorrhea and chlamydia.

Vaginal infections, such as vaginal dysbiosis and vaginal bacteriosis, may also be associated with miscarriage.

Cytomegalovirus (CMV) stems from the herpes virus and can be passed down from an infected mother to her baby, which can cause birth defects or future health problems. Pregnant people with CMV are also at a higher risk of miscarriage.

Naqvi also warns against consuming unpasteurized, or “raw,” milk, cheese, and other dairy products because of the risk of exposure to listeria bacteria. “Listeria is a reason for pregnancy loss,” Naqvi says.

Chronic conditions

Chronic pre-existing medical conditions can increase the risk of miscarriage, including high blood pressure, kidney disease, lupus and thyroid disorders.

People with existing Type 1 or 2 diabetes face an increased risk of miscarriage, as well. “They have more of a risk because of the swings in their blood sugar,” Holt says. “There’s also more risk of changes in fetal development related to diabetes.”

Environmental factors

Exposure to pollution, heavy metals, and other toxins may increase the risk of miscarriage. These are non-medical factors that can impact health, known as social determinants of health (SDOH).

“Where we work, where we live, where we play, how does that affect our reproductive health?” Naqvi says. “It’s huge, especially where we live. What are some of the exposures there? Are we living in a green space? What kind of air pollution, lead, arsenic, and types of food do we have access to?”

What are other less common causes?

Although they’re rare, these are other causes of miscarriage.

Immunological factors

The risk of miscarriage in people with certain autoimmune disorders can be high. Antiphospholipid syndrome (APS) can lead to pregnancy loss, particularly during the second and third trimesters, and can cause recurrent miscarriage.

Trauma

Trauma to the abdomen from accidents, violence, or falls can result in miscarriage.

Unknown causes

In some cases, a physician may not be able to determine the cause of a miscarriage.

What are the risk factors?

“There’s nothing typically that people have done wrong,” Holt says. “And often, there is no way to predict it, which is tough.”

However, certain factors may increase the risk of experiencing a miscarriage.

Previous miscarriages

People who’ve experienced one or more miscarriages can go on to have successful pregnancies. However, a prior miscarriage does increase the risk of having another one.

Multiple pregnancies

Being pregnant with more than one baby bumps up the risk of complications, including preterm labour and miscarriage. It’s not uncommon to lose one twin early in pregnancy (known as “vanishing twin syndrome”) without any symptoms or harm to the other baby.

Certain medical treatments

Some medications can increase the risk of pregnancy loss. These include common over-the-counter drugs such as aspirin and ibuprofen. Some prescription medications are safe during some trimesters, but not all. Before taking any OTC or prescription medications and supplements, ask your doctor.

Chemotherapy can cause miscarriage and is typically not administered until after 14 weeks gestation.

Lifestyle factors

Smoking, excessive alcohol consumption, drug use and obesity can increase the risk of miscarriage.

The consumption of marijuana more than about once a week has been found to negatively impact sperm quality, Holt says.

How can you prevent miscarriage?

There’s no guaranteed way to prevent a miscarriage. But, Dunne and other experts say, “Optimizing your health prior to pregnancy is the best thing you can do.”

Here are some recommendations:

Maintain a healthy weight

Being overweight or obese increases the risk of pregnancy loss. Mandelbaum recommends eating a healthy diet and exercising regularly.

Avoid harmful substances

Steer clear of tobacco, alcohol, and drugs. When possible, limit exposure to air pollution and other toxins.

Check with your healthcare provider to make sure your prescription medication and any supplements you’re taking are safe for conception and pregnancy or if you should stop taking them.

On the flip side, adding a prenatal vitamin that contains folic acid can prevent certain birth defects.

Manage chronic conditions

Getting chronic health conditions under control before getting pregnant, including high blood pressure, thyroid disorders, and diabetes, are essential to conceiving and having a healthy pregnancy.

Consult with your OB-GYN

Depending on your age and medical history, your physician may want to examine you more closely to spot potential problems ahead of time.

“It may also be beneficial to obtain a pelvic ultrasound before pregnancy to ensure that there are no anatomic abnormalities of the uterus prior to conceiving,” Mandelbaum says. “If a woman has experienced two prior miscarriages, a more detailed workup is necessary to see if there may be an identifiable cause.”

How can you seek support?

“Pregnancy is just wrought with emotions,” Holt says. “And there’s so much fear when you have a miscarriage, or you have two in a row. It’s typically just bad luck.”

If you’ve experienced miscarriage and all of the difficult feelings that can accompany it, know there are resources to help.

Your healthcare provider can likely connect you with a counsellor or social worker. “Seek a counsellor with experience in pregnancy loss,” Dunne says.

Many hospitals and fertility clinics have pregnancy loss support groups. Online support groups also exist, including Postpartum Support International. “They have support groups and information for grief and loss, as well as for all kinds of perinatal mental-health concerns,” Holt says.

Share your experience with the people in your life you’re closest to. “I encourage people to talk to friends and family about what they’re going through,” Dunne says. “Suffering in isolation can be really tough.”

Mandelbaum echoes this: “In my experience, I find that when you open the door to talking about miscarriage with your own personal friends or family, you will find you are far from alone. The realization that we don’t walk this path alone and sharing our experiences with miscarriage with one another can help tremendously.”

Experts

• Rachel Mandelbaum, M.D., FACOG, a board-certified OB-GYN and reproductive endocrinologist at HRC Fertility in California
• Fatima Naqvi, M.D., FACOG, an OB-GYN and interim medical director for women’s service at Atlantic Health System in New Jersey
• Sara Holt, CNM, MS, a certified nurse midwife at Byenveni Baby in Portland, Oregon
• Caitlin Dunne, M.D., FRCSC, a reproductive endocrinologist and fertility specialist in Vancouver