About 10 to 20 percent of known pregnancies end in miscarriage, and the single most common cause is a chromosomal problem in the embryo that happens by chance. Beyond genetics, a range of health conditions, anatomical differences, infections, and lifestyle factors can also play a role. In most cases, miscarriage is not caused by anything the parent did or failed to do.
Chromosomal Problems Are the Leading Cause
Roughly half of all miscarriages result from the embryo having extra or missing chromosomes, a category called aneuploidy. These errors typically occur randomly during fertilization or the earliest cell divisions, not because of anything inherited from either parent. The embryo simply cannot develop normally, and the pregnancy ends on its own, usually in the first trimester.
One of the most common specific errors is trisomy 16, where the embryo carries three copies of chromosome 16 instead of two. It affects an estimated 1 to 1.5 percent of recognized pregnancies and accounts for about 16 percent of first-trimester losses. Turner syndrome, in which the embryo has only a single X chromosome instead of two sex chromosomes, is another frequent finding. Only about 1 percent of pregnancies with Turner syndrome present at conception result in a live birth.
In a small number of couples who experience repeated losses, one partner carries what’s called a balanced translocation. Their own chromosomes are rearranged but contain all the necessary genetic material, so they’re healthy. When that rearrangement is passed to an embryo, though, it can result in extra or missing DNA. This accounts for about 5 percent of recurrent miscarriages.
How Maternal Age Affects Risk
Age is one of the strongest predictors of miscarriage, largely because egg quality declines over time, making chromosomal errors more likely during fertilization. A large study published by BMJ found the risk was lowest among women aged 25 to 29, at about 10 percent. It rose steadily after 30, reaching 53 percent among women 45 and older. This steep increase is driven almost entirely by the higher rate of chromosomal abnormalities in eggs as they age.
Health Conditions That Raise the Risk
Several chronic conditions can interfere with a pregnancy’s ability to sustain itself. Uncontrolled diabetes is one of the most well-documented. When blood sugar is poorly managed in early pregnancy, it disrupts the environment the embryo needs to develop. Thyroid disease, whether overactive or underactive, affects hormone levels that are critical for maintaining a pregnancy. Both conditions are manageable with treatment, which is why preconception care matters.
Hormonal imbalances more broadly can contribute. Progesterone, for example, helps prepare and maintain the uterine lining. If levels are too low, the lining may not support implantation or early growth. Conditions like polycystic ovary syndrome can affect ovulation and hormone balance in ways that increase miscarriage risk.
Obesity and being significantly underweight have both been linked to higher rates of pregnancy loss. Excess body weight can alter hormone levels and increase inflammation, while being underweight may signal nutritional deficiencies that affect early development.
Immune System and Blood Clotting Disorders
Antiphospholipid syndrome (APS) is an autoimmune condition that causes the blood to clot too easily. During pregnancy, these clots can form in the small blood vessels of the placenta, cutting off the blood supply the embryo depends on. Pathologists examining placentas from APS-related losses often find widespread tissue damage from poor blood flow. APS is one of the few treatable causes of recurrent miscarriage, and once identified, it can be managed with blood-thinning medication during pregnancy.
Uterine and Cervical Factors
The physical structure of the uterus plays a role in some losses. A septum (a wall of tissue dividing the uterine cavity), large fibroids pressing into the lining, or an unusually shaped uterus can all interfere with implantation or restrict the space and blood supply an embryo needs to grow.
Cervical insufficiency is a separate issue that typically causes losses later, in the second trimester. The cervix acts as a closed door between the uterus and the vagina. In cervical insufficiency, it softens and opens too early, before the fetus is viable. Nearly 25 percent of second-trimester miscarriages are attributed to this. Risk factors include prior cervical surgery, previous cervical damage during childbirth, or being born with a cervix that’s structurally weaker than typical. It can often be managed in future pregnancies with a stitch placed around the cervix to keep it closed.
Infections During Pregnancy
Certain infections can cross the placenta and directly harm a developing embryo or fetus. Listeria, a bacteria found in unpasteurized dairy and some deli meats, is particularly dangerous during pregnancy. Parvovirus B19 (the virus behind fifth disease) can cause severe anemia in a fetus. Cytomegalovirus (CMV), a common virus that causes mild cold-like symptoms in adults, can cause serious damage if a first infection occurs during pregnancy.
Toxoplasmosis, spread through undercooked meat or contact with cat feces, is another recognized risk. Several vaccine-preventable illnesses, including rubella, chickenpox, and measles, can also cause pregnancy loss if contracted during pregnancy. This is one reason immunity to these infections is checked before or early in pregnancy.
Caffeine, Smoking, and Alcohol
Lifestyle factors play a smaller role than chromosomal issues, but some carry measurable risk. A prospective study published in the American Journal of Obstetrics and Gynecology found that consuming 200 mg or more of caffeine per day (roughly two standard cups of coffee) more than doubled the risk of miscarriage compared to no caffeine. Even intake below 200 mg carried a moderately elevated risk.
Smoking during pregnancy restricts blood flow to the placenta and exposes the embryo to toxic chemicals, both of which increase the chance of loss. Alcohol consumption during early pregnancy is also associated with higher miscarriage rates, though pinning down a specific “safe” amount has proven difficult. Most guidelines recommend avoiding alcohol entirely during pregnancy for this reason.
The Father’s Age and Sperm Quality
Miscarriage risk isn’t determined solely by the pregnant person. Advancing paternal age is associated with declining sperm quality, including more DNA damage and a higher rate of chromosomal errors in sperm cells. These genetic abnormalities can contribute to embryos that don’t develop normally. The effect becomes more pronounced as men move into their 40s and beyond, mirroring the age-related increase seen in eggs, though the effect is less dramatic.
When Losses Happen Repeatedly
Most miscarriages are isolated events. A single loss, while devastating, does not typically indicate an underlying problem. Recurrent pregnancy loss, now defined as two or more miscarriages, prompts a more detailed investigation. Testing may look for chromosomal translocations in either partner, uterine abnormalities, thyroid function, blood clotting disorders like APS, and hormonal imbalances.
Even after multiple losses, the odds of a successful pregnancy remain favorable for most people. In many cases of recurrent loss, no specific cause is ever identified, and the next pregnancy proceeds normally. When a treatable cause is found, targeted management significantly improves the chances of carrying to term.
Timing Matters: Most Losses Are Early
Eight out of ten miscarriages happen in the first trimester, before the 12th week of pregnancy. The risk drops significantly once a heartbeat is detected on ultrasound, typically around week 6 or 7, and continues to fall with each passing week. By the start of the second trimester, the overall risk is substantially lower. Second-trimester losses, while they do occur, are far less common and more likely to involve structural or cervical causes rather than chromosomal ones.