What are Some of the Common Causes of Miscarriage?

If you’ve experienced pregnancy loss, there may be a hard-to-ignore question in the back of your mind: what, exactly, causes miscarriage?

You may know that it’s common—as many as 1 in 4 pregnancies end in miscarriage, and the real number is, unfortunately, even higher when you factor in unknown pregnancies—but as frequently as it happens, many prospective parents still don’t know what actually causes it.

That giant question mark can make the miscarriage experience even worse. Grieving over your pregnancy loss is hard enough, but when you don’t know where to place the blame, and wonder if it could happen to you again, you end up facing fear, anger, and frustration on top of grief.

When miscarriage happens, it’s crushing. But the thing to keep in mind is this: miscarriage is not your fault, and there’s nothing you could have done to prevent it or change it. The universe has a pretty messed up way of working, huh?

So...what are the causes of miscarriage, anyway? And why do they happen to so many women?

Here are some common reasons why miscarriage can occur, and what you should ask your doctor if it’s happened to you.

Possible miscarriage cause #1: abnormal chromosomes

Biology may be a science, but it kind of functions like a delicate musical instrument: one wrong note and the whole thing goes out of tune. Translated to genetics, this means that if one teensy piece of the babymaking chromosomal puzzle doesn’t fall perfectly into place, the embryo may not develop properly, potentially causing miscarriage.

According to OBGYN Mary Jane Minkin, M.D., clinical professor of obstetrics, gynecology and reproductive sciences at Yale University School of Medicine, an embryo that’s genetically abnormal is the most common cause of miscarriage. She says sometimes this abnormality occurs at the very start of the fertilization process and other times it happens a bit later.

Either way, you can’t control the genetic processing that happens when sperm meets egg (unless you did in vitro fertilization and your doctor tested the genes of your embryo before transferring it, which many do!).

You can, however, ask your doctor to do some detective work for you if you’ve had repeated miscarriages: placental and fetal tissue can be tested for chromosomal abnormalities, giving you a clearer picture of what’s going on in your body when a fertilized egg ends in miscarriage.

Possible miscarriage cause #2: advanced maternal age

You know what sucks? Because women are born with all the eggs they’ll ever have, those eggs age right along with us...and can increase your chances of those chromosomal abnormalities we mentioned.

In a 2019 study published in the British Medical Journal, the risk of miscarriage rose sharply in women over the age of 30, reaching as high as 53 percent by age 45.

Even men, who generate fresh sperm all the time, are subject to chromosomal aging, says Minkin: “Guys keep making new sperm, although there is data to show that older fathers do have more genetic issues, too.”

If you and your partner are concerned about your genetic health—whether it’s because of aging or not—you can ask your doctor for genetic screening, which may alert you to risk factors you otherwise wouldn't know about.

Possible miscarriage cause #3: infertility or hormonal issues

This is going to sound like a chicken vs. egg scenario, but hear us out: infertility issues may actually cause...more infertility issues? Basically, your miscarriage rate can be higher if you’ve struggled to conceive in the past, possibly because there’s something up with your hormones at the root of your infertility challenges.

“Occasionally, a woman might not be ovulating well enough, [meaning] she is ovulating enough to produce the egg, but not enough to make the progesterone needed from the ovary to maintain the pregnancy,” explains Minkin. This might apply to you if you experience anovulatory cycles, irregular cycles, or if you have polycystic ovarian syndrome (PCOS).

Possible miscarriage cause #4: infection or chronic illness

No, we’re not talking about colds or stomach bugs here—we’re talking about more severe infections, like sexually transmitted diseases (STDs), cytomegalovirus (CMV), and pelvic inflammatory disease. We’re also talking about chronic conditions, many of which come with the one-two punch of increasing your risk of miscarriage and infertility issues.

Kecia Gaither, M.D., double board-certified physician in OB/GYN and maternal fetal medicine and director of perinatal services at NYC Health + Hospitals/Lincoln, says maternal illnesses like diabetes, thyroid disorders, autoimmune disorders such as lupus, and blood clotting disorders can contribute to the overall causes of miscarriage, too.

Possible miscarriage cause #5: reproductive or anatomical issues

There are a bunch of congenital abnormalities in the reproductive system that can a) make it harder for you to conceive and b) make it harder for a fertilized embryo to thrive after conception.

Some of these abnormalities include:

• a misshapen uterus (like a double- or half-uterus)
• uterine fibroids
• a uterus with a septum
• blocked or damaged fallopian tubes
• endometriosis scarring

You may know about these anomalies already if you’ve got ‘em; they may have affected your menstruation or caused other symptoms. Either way, many of them can be treated if they’re contributing to infertility, so talk to your doctor.

Possible miscarriage cause #6: substance abuse

According to Dr. Gaither, frequent drug and alcohol use may also increase your chance of miscarriage. No, we don’t mean the celebratory glass of champagne you had on your birthday before you knew you were pregnant. It’s the consistent consumption of alcohol—especially as you move past week five of pregnancy—that’s affiliated with miscarriage. (And any amount of drug use, at any point during pregnancy, is potentially a problem.)

Addiction is a debilitating mental health condition; if you’re struggling, consider seeking help—especially if you’re trying to have a baby.

What to ask your doctor

If this is your first miscarriage, it may not be necessary to ask your doctor anything just yet; Dr. Minkin says miscarriage is common enough that it doesn’t always mean there are overarching fertility issues that need to be addressed. On the other hand, if you’ve been trying to conceive for a while, have a known fertility issue, or received any kind of fertility treatment, you may want to investigate any underlying issues as soon as possible rather than wait.

It’s best to talk to your doctor about a miscarriage so they are aware it happened and can make a decision, based on your overall health, about how to proceed. If your doctor feels more evaluation is needed to determine what caused your miscarriage, Dr. Gaither says there are a few things your doctor can do in terms of getting your fertility prospects checked out. These may include:

• Having miscarriage tissue genetically evaluated
• Having diagnostic tests to look for uterine or cervical anomalies
• Managing any other medical conditions that could be contributing to your inability to maintain a pregnancy

If you’re grieving a pregnancy loss, we know this information is probably pretty overwhelming. You may not be eager to dig into the reason behind your miscarriage at this point — and that’s understandable. You should take the time you need to consider your options and move forward when you feel ready.

But you should also know that miscarriage doesn’t mean you can’t ever get pregnant. Many women go on to have healthy pregnancies after experiencing loss, sometimes naturally and sometimes with assistance from a fertility specialist. We’re rooting for you.

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Read more in Does Using Donor Eggs Decrease the Risk for Miscarriage?