How Does Miscarriage Happen? Causes And Risks

Miscarriage is the loss of a pregnancy before 20 weeks. It happens most often because the fertilized egg or developing embryo has chromosomal abnormalities that make normal growth impossible. When that happens, the pregnancy stops developing, hormone levels fall, and the uterus eventually empties — sometimes on its own, sometimes with medical or surgical help.

It is the most common complication of early pregnancy. Research consistently shows that roughly 10 to 20 percent of known pregnancies end in miscarriage, and the true number is likely higher because many losses happen before a person knows they are pregnant. Most occur in the first 12 weeks.

What follows is an honest look at how miscarriage actually happens, what causes it, what raises risk, and what does not.

How Does Miscarriage Happen? Causes And Risks

Miscarriage happens when a pregnancy stops developing and the body begins to expel the pregnancy tissue. The trigger is usually a problem that occurred at or near conception — not something the pregnant person did.

In the earliest weeks, the most common cause is a chromosomal abnormality in the embryo. Chromosomes carry the genetic instructions for development. When an embryo has too many or too few chromosomes, or a structural problem in a chromosome, development stalls. The body recognizes this and the pregnancy ends.

Chromosomal errors are typically random events in the sperm or egg. They are not inherited from either parent in most cases, and they are not caused by stress, exercise, or a single cup of coffee.

Less often, miscarriage is linked to:

• Problems with the uterus, such as fibroids or an unusually shaped uterus
• An infection in the reproductive tract
• Uncontrolled thyroid disease or uncontrolled diabetes
• Certain blood-clotting disorders
• Hormonal conditions such as polycystic ovary syndrome, though the evidence here is mixed

For many people who miscarry, no specific cause is ever identified. That is common and does not mean anything was missed.

What Actually Happens in the Body During a Miscarriage

A miscarriage is a process, not a single event. It unfolds over hours to days, and the experience varies widely.

When a pregnancy stops developing, the body’s hormone levels — including hCG and progesterone — begin to drop. Without hormonal support, the lining of the uterus starts to break down. The cervix may soften and open. The uterus then contracts to push out the pregnancy tissue.

What a person notices depends on how far along the pregnancy was:

• Bleeding — often the first sign, ranging from light spotting to heavy flow with clots
• Cramping — similar to strong menstrual cramps, sometimes more intense
• Passing tissue — this can be visible, and it can be unsettling
• Loss of pregnancy symptoms — such as nausea or breast tenderness fading

Sometimes the pregnancy stops developing but the body does not expel it. This is called a missed miscarriage. There may be no bleeding or cramping at all, and the loss is found on an ultrasound. Some clinicians recommend waiting to see if the body completes the process; others recommend medication or a procedure called dilation and curettage. Each option has trade-offs, and the right choice depends on the person’s health, preference, and how far along the pregnancy was.

What Are the Main Causes of Miscarriage?

Chromosomal abnormalities account for the majority of miscarriages that happen in the first trimester. This is the single most established fact in the field.

Beyond that, causes fall into a few categories:

Genetic causes

Random chromosomal errors in the embryo. These become more common with age, which is one reason miscarriage rates rise as people get older.

Anatomical causes

Fibroids, a septum inside the uterus, or an unusually shaped uterus can interfere with implantation or blood supply. These are less common but treatable in some cases.

Medical conditions

Poorly controlled diabetes and thyroid disease are linked to higher miscarriage risk. So are certain autoimmune conditions, such as antiphospholipid syndrome, which affects blood clotting.

Infection

Certain infections can reach the uterus or placenta and disrupt a pregnancy. These are uncommon causes of miscarriage in the United States.

What is not a cause: exercising, having sex, lifting objects, working, or experiencing a sudden scare. These do not cause miscarriage. The evidence on this is strong and consistent.

What Raises the Risk of Miscarriage?

Age is the strongest known risk factor. The chance of miscarriage rises steadily through the 30s and more sharply after 35. This reflects the higher rate of chromosomal errors in eggs as people age.

Other factors that raise risk include:

• A previous miscarriage — risk goes up somewhat with each prior loss
• Smoking, alcohol use, and use of certain recreational drugs during pregnancy
• Uncontrolled diabetes or thyroid disease
• Being significantly underweight or overweight
• Certain uterine or cervical conditions
• Some medications, including certain acne treatments and some blood pressure drugs

Two or more miscarriages in a row is called recurrent pregnancy loss. It affects a small percentage of people who miscarry. When it happens, clinicians typically recommend testing to look for chromosomal, anatomical, hormonal, or clotting causes. Even after testing, no cause is found in a substantial share of cases.

One clarification worth making: a single miscarriage is not a sign that something is wrong with you or that you cannot carry a pregnancy. Most people who miscarry once go on to have a healthy pregnancy.

Can Miscarriage Be Prevented?

Most early miscarriages cannot be prevented. The cause is usually a chromosomal error that happens at conception, and there is no known way to stop it.

That said, some steps reduce risk in specific situations:

• Managing diabetes and thyroid disease before and during pregnancy
• Avoiding smoking, alcohol, and recreational drugs
• Reviewing medications with a clinician before trying to conceive
• Treating certain uterine problems, such as a septum, if found

No diet, supplement, or lifestyle change has been shown to prevent miscarriage caused by chromosomal errors. Claims otherwise are not supported by clinical evidence.

If you have had recurrent losses, ask your clinician about testing. In some cases, a treatable cause is found and addressed.

When to Seek Medical Care

Heavy bleeding, severe pain, fever, or passing large clots during pregnancy needs prompt medical attention. So does any bleeding that concerns you, even if it seems light.

Some warning signs to take seriously:

• Bleeding that soaks through a pad in an hour or less
• Fever above 100.4°F (38°C)
• Severe or worsening abdominal pain
• Foul-smelling discharge
• Dizziness or fainting

These can signal an incomplete miscarriage or an infection, both of which need treatment. Do not wait to see if things improve on their own.

What Happens After a Miscarriage

Physical recovery usually takes a few days to a few weeks. Bleeding may continue for up to two weeks, sometimes with light spotting after that. Cramping typically eases within a few days.

Emotional recovery does not follow a timeline. Grief after miscarriage is real and can be intense, even if the pregnancy was early. Some people feel better within weeks; others carry it much longer. Both are normal.

Most clinicians recommend waiting until bleeding has stopped and you feel physically ready before trying to conceive again. Some research suggests that conceiving within six months of a miscarriage may be associated with lower risk of another loss, but the evidence is not strong enough to make this a firm recommendation. Talk with your clinician about what is right for your situation.