The word “abortion” covers two distinct events: a spontaneous abortion (miscarriage), where a pregnancy ends on its own, and an induced abortion, where a pregnancy is ended intentionally through medication or a procedure. Both have well-understood causes, and understanding them can help you make sense of what happened or what to expect.
Chromosomal Problems: The Leading Cause of Miscarriage
Chromosomal abnormalities cause about 50% of all miscarriages in the first trimester. These are random errors that occur when the fertilized egg divides, resulting in too many or too few chromosomes. An embryo with the wrong number of chromosomes typically cannot develop normally, and the pregnancy ends, often before the person even knows they’re pregnant. This type of loss is not caused by anything the parent did or didn’t do.
Because these errors are largely random, they can happen to anyone. However, they become more common with age, which is one reason miscarriage rates climb significantly after 35.
How Maternal Age Affects Risk
Age is one of the strongest predictors of miscarriage. The numbers shift dramatically across a woman’s reproductive years:
- Ages 20 to 30: 9% to 17% chance of miscarriage
- Age 35: 20% chance (1 in 5)
- Age 40: 40% chance (4 in 10)
- Age 45: 80% chance (8 in 10)
The steep increase after 40 is driven primarily by the rising rate of chromosomal errors in eggs as they age. This doesn’t mean pregnancy at 40 is impossible or inadvisable, but it does mean the statistical odds of loss are meaningfully higher.
Medical Conditions That Raise Risk
Several maternal health conditions can increase the likelihood of pregnancy loss. Uncontrolled diabetes is one of the most significant. When blood sugar stays elevated during early pregnancy, it disrupts the environment the embryo needs to develop. Thyroid disease, both overactive and underactive, can interfere with the hormonal balance that sustains a pregnancy. Obesity independently raises miscarriage risk as well.
Infections during pregnancy can also trigger a loss, as can hormonal imbalances that affect progesterone or other hormones critical to maintaining the uterine lining. Autoimmune conditions, where the immune system mistakenly attacks the body’s own tissues, have been linked to recurrent miscarriage in some people.
Structural Problems With the Uterus or Cervix
Sometimes the issue is physical. An irregularly shaped uterus, uterine fibroids, or scar tissue inside the uterus can prevent an embryo from implanting properly or growing as it should.
Cervical insufficiency is another structural cause, and it plays a particularly important role later in pregnancy. This happens when the cervix opens, weakens, or shortens too early, often without any pain or contractions. It accounts for nearly 25% of second-trimester miscarriages and occurs in about 1 in 100 pregnancies. Risk factors include previous cervical surgery, injury during a prior birth, or a genetic connective tissue disorder like Ehlers-Danlos syndrome. Because it often produces no warning symptoms, it’s sometimes not identified until after a loss has already occurred.
Smoking, Alcohol, and Caffeine
Smoking has a clear, causal link to pregnancy loss. A large genetic analysis found that a predisposition to smoking increased the odds of miscarriage by about 31%. This wasn’t just a correlation driven by other lifestyle factors. The genetic data pointed to smoking itself as the driver.
The picture for alcohol and coffee is less alarming than many people assume. The same study found no significant association between moderate alcohol or coffee consumption and pregnancy loss when analyzed through genetic data. That said, most health organizations still recommend limiting caffeine and avoiding alcohol during pregnancy as a precaution, since other types of harm to a developing fetus remain a concern even if miscarriage risk specifically isn’t elevated.
What Does Not Cause Miscarriage
Many people blame themselves after a loss, wondering if something they did triggered it. The medical evidence is reassuring on several fronts. Exercise during pregnancy does not cause miscarriage. Neither does sexual intercourse. Short-term, everyday stress doesn’t raise the risk either.
Chronic, sustained stress or sudden extreme stress may modestly increase miscarriage risk, but the key point remains: most of the time, there is nothing a pregnant person does that causes a miscarriage. Bed rest, pelvic rest, vitamins, and other historically recommended interventions have not been proven to prevent early pregnancy loss.
How Induced Abortion Works: Medication
An induced abortion is a deliberate decision to end a pregnancy, and it’s accomplished through either medication or a procedure. The approach depends largely on how far along the pregnancy is.
Medication abortion uses two drugs taken in sequence and is approved for pregnancies up to 70 days (10 weeks) from the last menstrual period. The first drug stops the supply of progesterone, the hormone that maintains the uterine lining. Without it, the uterus can no longer sustain the pregnancy. Twenty-four to 48 hours later, a second medication causes the uterus to contract and expel its contents, similar to a heavy period with cramping. The process typically takes a few hours to a day after the second medication is taken, though bleeding can continue for one to two weeks.
How Induced Abortion Works: Procedures
For pregnancies in the first trimester, a suction aspiration (also called vacuum aspiration) is the standard approach. It uses gentle suction to empty the uterus and is typically completed in a single visit lasting 10 to 15 minutes.
For second-trimester pregnancies (weeks 14 through 26), a procedure called dilation and evacuation, or D&E, is used. This involves dilating the cervix and using a combination of suction and instruments to remove the pregnancy tissue. D&E is performed not only for elective abortions but also to manage miscarriages, pregnancies with serious fetal abnormalities, or situations where the pregnant person’s health is at risk. Surgical evacuation has a success rate approaching 99%.
Spontaneous vs. Induced: The Key Difference
The essential distinction is control. Spontaneous abortion is something that happens to you, driven by genetic errors, health conditions, or structural problems that are largely outside your influence. Induced abortion is something you choose, carried out through well-established medical methods. Both are common. Roughly 10% to 20% of known pregnancies end in miscarriage, and the true number is likely higher since many losses happen before a pregnancy is even detected.
If you’ve experienced a pregnancy loss and are trying to understand why, the most likely explanation, especially in the first trimester, is a chromosomal abnormality that no action could have prevented. For recurrent losses (three or more), evaluation for underlying conditions like thyroid disease, uterine abnormalities, or clotting disorders can sometimes identify a treatable cause.