A miscarriage happens when a pregnancy ends on its own before the 20th week, and the most common reason is that the embryo received the wrong number of chromosomes at conception. Most miscarriages occur during the first 13 weeks. About 10 to 20 percent of known pregnancies end this way, though the true number is likely higher because many losses happen before a person even realizes they’re pregnant.
Chromosomal Errors Are the Leading Cause
When a sperm fertilizes an egg, the two cells combine their genetic material. Normally, this produces an embryo with exactly 46 chromosomes. But errors during cell division can leave the embryo with too many or too few chromosomes, and most of those embryos cannot develop normally. A large study of over 7,000 miscarriage cases found chromosomal abnormalities in about 67 percent of them. Other research puts the range between 40 and 76 percent of first-trimester losses.
The most common errors involve extra copies of specific chromosomes, particularly chromosomes 13, 15, 18, 21, and 22. These aren’t inherited conditions or the result of anything either parent did. They’re random mistakes in cell division that become more frequent as egg cells age, which is a major reason miscarriage risk climbs with maternal age.
What Happens Inside the Body
When an embryo stops developing, hormone levels that sustain the pregnancy begin to drop. Progesterone, the hormone responsible for maintaining the uterine lining, falls. Without it, the lining begins to break down and the body starts the process of passing the pregnancy tissue.
Sometimes the embryo stops growing days or even weeks before any outward signs appear. This is called a missed miscarriage, where an ultrasound reveals no heartbeat but the body hasn’t yet responded. In other cases, the process begins quickly. The uterus contracts to expel tissue, which is what causes the cramping that many people describe as similar to intense period pain or early labor contractions.
Physical Signs and How It Feels
The symptoms typically follow a recognizable pattern:
- Vaginal bleeding, which may start as light spotting and progress to heavier flow with clots
- Cramping or pain in the pelvis, abdomen, or lower back
- Passing tissue or fluid from the vagina
- Rapid heartbeat, particularly if bleeding is heavy
The intensity varies widely depending on how far along the pregnancy is. A loss at five or six weeks may feel like a heavy, painful period. A loss at 10 or 12 weeks typically involves more bleeding, stronger cramps, and identifiable tissue. Some people pass everything within a few hours. For others, the process unfolds over several days, with bleeding that starts and stops before completing.
Not all bleeding in early pregnancy means a miscarriage is happening. Light spotting occurs in many healthy pregnancies. Heavy bleeding with cramping pain is a more urgent sign.
Risk by Week of Pregnancy
The risk of losing a pregnancy is not constant. It’s highest in the earliest weeks and drops sharply as the pregnancy progresses. A pooled analysis of over 12,000 pregnancies tracked the weekly risk from the last menstrual period:
- Weeks 5 to 7: 3 to 4 percent per week
- Weeks 8 to 9: 3 to 8 percent per week
- Weeks 10 to 11: 6 to 7 percent per week
- Week 12: about 1 percent
- Weeks 13 to 19: less than 0.5 percent
Once a heartbeat is visible on ultrasound (usually around week 6 or 7), the overall risk drops considerably. By the end of the first trimester, the chance of loss is very low.
How Maternal Age Affects Risk
Age is one of the strongest predictors of miscarriage because older eggs are more likely to have chromosomal errors during division. The risk increases gradually through the 30s and then accelerates:
- Ages 20 to 30: 9 to 17 percent
- Age 35: about 20 percent
- Age 40: about 40 percent
- Age 45: about 80 percent
The proportion of miscarriages caused by multiple chromosomal errors and by specific trisomies (extra copies of chromosomes 13, 18, 21, and 22) rises steadily with maternal age. This is a biological reality of egg cell aging, not something that can be prevented through lifestyle changes.
Other Causes Beyond Chromosomes
While chromosomal problems account for the majority of first-trimester losses, other factors play a role, especially in later miscarriages or recurrent losses.
Health Conditions
Uncontrolled diabetes, thyroid disorders, and immune system conditions like antiphospholipid syndrome can all interfere with a pregnancy’s ability to sustain itself. Hormonal imbalances, particularly insufficient progesterone production, can prevent the uterine lining from supporting an embryo. Infections of the uterus or cervix can also trigger pregnancy loss. Many of these conditions are treatable, which is why they’re investigated after repeated miscarriages.
Uterine Structure
Some people are born with a uterus that has an unusual shape, and these structural differences can contribute to pregnancy loss. A septate uterus, where a wall of tissue divides the inside of the uterus into two cavities, is the most common type, accounting for about 35 percent of congenital uterine anomalies. The embryo may implant on the septum, which has poor blood supply, and fail to grow. Large fibroids (noncancerous growths in the uterine wall) can also distort the uterine cavity enough to interfere with implantation or blood flow to the placenta. These structural issues are more often linked to recurrent miscarriage than to a single isolated loss.
Stress and Lifestyle
Data from the National Women’s Health Study found a clear link between stressful life events, a stressful job situation, and feelings of anxiety and depression on the one hand, and miscarriage risk on the other. The biological pathway likely involves stress hormones that affect blood flow to the uterus and immune function during early pregnancy. Smoking and heavy alcohol use are also established risk factors. Moderate exercise and normal daily activity do not cause miscarriage.
What Happens After a Miscarriage
When a miscarriage is confirmed, there are generally three paths forward. Expectant management means waiting for the body to pass the pregnancy tissue on its own, which happens successfully in the majority of cases within a few weeks. Medical management uses medication to speed the process. Surgical management involves a brief procedure to remove tissue from the uterus, which is sometimes recommended if bleeding is heavy or if tissue remains after the other approaches.
Research comparing expectant and medical management shows similar outcomes. One systematic review found no significant difference between the two approaches at three to four weeks, with both achieving completion in roughly 57 to 62 percent of cases. The choice often comes down to personal preference and how quickly someone wants the physical process to be over.
Physically, bleeding typically continues for one to two weeks after a miscarriage, though it may be lighter. Most people get their next period within four to six weeks. Fertility usually returns quickly, and many people ovulate as soon as two weeks after a loss.
Recurrent Miscarriage
A single miscarriage is extremely common and usually does not indicate an underlying problem. Recurrent pregnancy loss, currently defined as two or more losses, affects a smaller group and warrants investigation. Testing may include bloodwork for hormonal or clotting disorders, imaging of the uterus to check for structural issues, and chromosomal analysis of both partners to look for balanced translocations (rearrangements of genetic material that don’t affect the parent but can produce unviable embryos).
Even after two losses, the odds of a successful next pregnancy remain high for most people. Identifying and treating an underlying cause, when one exists, improves those odds further.