Most miscarriages happen because the embryo develops with the wrong number of chromosomes, making it unable to grow into a viable pregnancy. About 50% of all first-trimester losses trace back to these random genetic errors. The loss is not typically caused by anything the pregnant person did or didn’t do. Understanding the biological chain of events, from what goes wrong at the cellular level to what happens physically, can help make sense of an experience that often feels sudden and confusing.
What Goes Wrong at the Cellular Level
When a sperm fertilizes an egg, two sets of chromosomes combine to form the blueprint for a new embryo. Sometimes the egg or sperm carries more or fewer chromosomes than it should, giving the embryo an abnormal total. As that fertilized egg begins dividing and multiplying into millions of cells, errors can also arise during the copying process itself. Either way, the result is an embryo that can’t develop the structures it needs to survive.
These chromosomal problems are largely random. They become more common with age, which is one reason miscarriage risk rises sharply over time. Data from a Danish study of over 1.2 million pregnancies shows a roughly fourfold increase in miscarriage risk between ages 20 and 40. Paternal age plays a role too, though less dramatically. None of this reflects a flaw in either parent’s health. It reflects the inherent imprecision of cell division, a process that becomes less reliable as reproductive cells age.
When the Body Recognizes a Non-Viable Pregnancy
A healthy pregnancy depends on a steady supply of progesterone, the hormone that maintains the uterine lining and supports the embryo’s blood supply. When an embryo stops developing, the tissue that would eventually become the placenta stops producing the hormonal signals needed to sustain the pregnancy. Progesterone levels drop. Without that hormonal support, the thickened uterine lining begins to break down, much like it does at the start of a menstrual period, but typically with heavier bleeding and more intense cramping.
The uterus then begins contracting to expel the pregnancy tissue. This can happen quickly over a few hours or unfold gradually over days or even weeks. Some people notice spotting first, followed by heavier bleeding with clots and tissue. Others experience strong, wave-like cramping that intensifies before the tissue passes. The physical process varies widely from person to person and pregnancy to pregnancy.
Health Conditions That Raise the Risk
While chromosomal errors account for the majority of early losses, certain maternal health conditions increase the likelihood of miscarriage. Uncontrolled diabetes and thyroid disease can disrupt the hormonal environment a pregnancy needs. Infections can interfere with implantation or damage the developing embryo. Obesity and being significantly underweight both affect risk: women with a BMI below 18.5 at the start of pregnancy have a 75% higher risk compared to those in a normal BMI range.
Structural issues with the uterus also matter. A septate uterus, where a wall of tissue divides the uterine cavity, is linked to higher rates of both first and second trimester loss. Other congenital uterine differences can restrict blood flow to the embryo or limit the space available for growth. These conditions can also increase the risk of preterm birth and growth restriction in pregnancies that do continue.
What Doesn’t Cause Miscarriage
Many people who miscarry search their recent behavior for an explanation. The evidence is reassuring on several fronts. A large UK study called the Nationwide Women’s Health Study found no link between miscarriage and prolonged standing, lifting heavy objects, strenuous exercise, or working during pregnancy. The classic occupational physical stressors that people worry about simply don’t show up as risk factors in well-designed research.
Caffeine is another common concern. That same study found what initially looked like a dose-response relationship between caffeine and miscarriage risk, but the association disappeared once researchers accounted for pregnancy nausea. Women with healthy pregnancies tend to feel more nauseated and therefore drink less coffee, creating the illusion that caffeine was protective at lower doses. The independent effect wasn’t there.
Factors that do have real evidence behind them include smoking, alcohol use, a previous history of miscarriage (which roughly doubles the risk in subsequent pregnancies), and significant psychological stress. Having a previous live birth, on the other hand, reduces the risk of future miscarriage by about 40%.
How Risk Changes Week by Week
Miscarriage risk is highest in the earliest weeks of pregnancy and drops significantly as the pregnancy progresses. Overall, early miscarriage occurs in an estimated 10 to 20% of recognized pregnancies. Once a heartbeat is visible on ultrasound around 6 to 7 weeks, the risk falls to roughly 10%. A study of over 300 women with a history of recurrent loss found that seeing a heartbeat at 8 weeks meant a 98% chance of the pregnancy continuing, and at 10 weeks that figure rose to 99.4%.
Second trimester miscarriage, sometimes called late miscarriage, is much rarer. It occurs in about 3 to 4% of pregnancies. Losses after the first trimester are more likely to involve structural uterine issues, cervical problems, or infection rather than chromosomal errors.
How a Miscarriage Is Confirmed
When bleeding or cramping raises concern, an ultrasound is the primary tool for determining whether a pregnancy is still viable. Doctors look for specific markers. If the embryo measures 7 millimeters or longer and has no heartbeat, the pregnancy has failed. If the gestational sac reaches 25 millimeters in diameter with no visible embryo inside, that’s also definitive. Measurements below those thresholds are considered suspicious but not diagnostic, and a follow-up scan is typically scheduled one to two weeks later to be certain.
This waiting period exists because early pregnancies can be hard to date precisely. An embryo that appears too small might simply be a few days younger than expected. The diagnostic criteria are deliberately conservative to avoid ever misidentifying a viable pregnancy as a loss.
What Happens After a Miscarriage Is Diagnosed
There are three main paths forward, and in many cases you’ll have a choice among them.
- Waiting for the process to happen naturally. If there are no signs of infection, you can let your body pass the pregnancy tissue on its own. This often happens within two weeks of the embryo stopping development but can take up to eight weeks. It’s most commonly offered in the first trimester.
- Medication. A combination of two medications helps the uterus contract and release the remaining tissue. The combination approach is more effective than a single medication alone and reduces the likelihood of needing a follow-up procedure.
- A minor surgical procedure. A procedure called suction dilation and curettage involves gently opening the cervix and removing tissue from the uterus. It’s quick and is sometimes recommended when the other options haven’t fully worked or when there’s a medical reason to complete the process sooner.
All three approaches are safe and well-established. The right choice depends on how far along the pregnancy was, your medical history, and your own preference for how you want to manage the experience physically and emotionally.
Recurrent Miscarriage
Recurrent pregnancy loss is currently defined as two or more miscarriages before 20 weeks. The losses don’t need to be consecutive. After two or more losses, testing is typically offered to look for underlying causes like clotting disorders, hormonal imbalances, uterine structural issues, or chromosomal differences in either partner that increase the odds of producing embryos with abnormal chromosome counts. In many cases, though, no specific cause is found, and the next pregnancy goes on to succeed.