The most common cause of first-trimester miscarriage is a chromosomal abnormality in the embryo, accounting for roughly 50% of all pregnancy losses before 13 weeks. In most cases, there is nothing a parent did or could have done to prevent it. The remaining causes range from underlying health conditions and uterine structure to immune system disorders and infections, though sometimes no specific cause is ever identified.
Chromosomal Abnormalities
About half of all first-trimester miscarriages happen because the embryo received an abnormal number of chromosomes at conception. If either the egg or sperm carries too many or too few chromosomes, the resulting embryo can’t develop normally. The pregnancy may implant and begin growing, but it stops progressing, often before a heartbeat is ever detected. This is essentially a random biological error during cell division, and it becomes more likely as egg quality declines with age.
These chromosomal problems are not inherited conditions in most cases. They’re one-time events that don’t typically repeat in future pregnancies. This is why the vast majority of people who experience a single miscarriage go on to have a healthy pregnancy afterward, with the risk of miscarriage in the next pregnancy sitting around 20%.
Maternal Age and Egg Quality
Age is one of the strongest predictors of miscarriage risk, and the numbers shift dramatically after 35. For people between 20 and 30, the chance of miscarriage is roughly 9% to 17%. At 35, it rises to about 20%. By 40, the risk reaches 40%, and at 45, it climbs to around 80%.
The reason is straightforward: eggs accumulate more chromosomal errors over time. The longer eggs have been stored in the ovaries, the more likely they are to divide unevenly when fertilized. This is the same mechanism behind the chromosomal abnormalities described above, just amplified by age.
Thyroid Problems and Diabetes
An underactive thyroid that isn’t well controlled during early pregnancy raises miscarriage risk significantly. The Endocrine Society recommends keeping thyroid hormone levels in a specific range during the first trimester. Women whose levels fall moderately outside that range have about 1.8 times the normal risk of miscarriage, while those with more significantly elevated levels face nearly 4 times the risk. If you have a known thyroid condition, getting your levels checked and adjusted before or early in pregnancy can make a real difference.
Uncontrolled diabetes works through a different pathway but has a similar effect. Persistently high blood sugar creates a hostile environment for a developing embryo, interfering with normal cell growth in the earliest weeks. Well-managed diabetes, on the other hand, does not carry the same elevated risk. The key factor is blood sugar control around the time of conception and in the weeks that follow.
Uterine and Structural Issues
The shape and condition of the uterus plays a direct role in whether a pregnancy can sustain itself. A septate uterus, where a wall of tissue divides the uterine cavity, is the most common structural abnormality present from birth and is linked to recurrent miscarriage. Other congenital variations include a heart-shaped uterus (bicornuate), a half-formed uterus (unicornuate), and a double uterus (didelphys). Each of these can reduce the space available for an embryo to implant and grow.
Fibroids, which are noncancerous growths in or on the uterus, can also contribute. The ones most likely to cause problems are those that grow into the uterine wall (intramural) or bulge into the uterine cavity itself (submucosal). If large enough, these growths can block implantation or prevent the uterine lining from supporting a pregnancy. Scar tissue from previous surgeries or infections can have a similar effect, damaging the lining and making it harder for a pregnancy to take hold.
Immune System Disorders
Antiphospholipid syndrome is an autoimmune condition where the immune system produces antibodies that trigger abnormal blood clotting and inflammation. In pregnancy, this leads to poor blood flow to the placenta, starving the embryo of oxygen and nutrients. The condition is specifically associated with recurrent early losses, and doctors typically consider it after three or more consecutive miscarriages before 10 weeks.
Diagnosis involves blood tests for specific antibodies combined with a history of pregnancy complications. The condition is treatable, and people diagnosed with it often have successful pregnancies with appropriate management. If you’ve experienced multiple early losses, this is one of the conditions your doctor will likely screen for.
Infections
Certain infections during the first trimester can directly harm the developing embryo or interfere with its blood supply. Parvovirus B19 (the virus that causes “fifth disease” in children) can cause severe anemia in the fetus and, in some cases, miscarriage when infection occurs early in pregnancy. Listeria, typically contracted through contaminated food, can cross the placenta and cause pregnancy loss. Toxoplasmosis, spread through undercooked meat or contact with cat feces, poses a similar risk.
These infections don’t always cause harm. Many people are exposed without any complications. But first-trimester exposure carries a small, measurable increase in risk, which is why food safety guidelines during pregnancy focus heavily on avoiding raw or undercooked meats, unpasteurized dairy, and deli meats.
Caffeine and Substance Use
Caffeine intake above 200 milligrams per day (roughly two standard cups of coffee) is associated with a meaningful increase in miscarriage risk. A large prospective study found that people consuming 200 mg or more daily had about 2.2 times the risk of miscarriage compared to those who consumed none. Even intake below 200 mg showed a trend toward increased risk, though the finding wasn’t statistically conclusive.
Smoking, alcohol, and recreational drug use all raise the risk as well. Smoking restricts blood flow to the placenta, alcohol disrupts fetal development at the cellular level, and cocaine and methamphetamine can cause placental separation. These are modifiable risks, meaning reducing or eliminating them genuinely lowers the chance of loss.
What Doesn’t Cause Miscarriage
Exercise, including strenuous or high-impact workouts, does not cause miscarriage. Most studies find no link, and moderate activity like walking or swimming may actually reduce complications later in pregnancy. Even weightlifting and intense cardio are typically safe for physically fit people with healthy pregnancies.
Sex during pregnancy does not increase the risk of early miscarriage. Neither does working a regular job, feeling stressed, or experiencing an emotional shock. These beliefs persist because miscarriage is common (affecting roughly 10% to 20% of known pregnancies), and it’s human nature to search for an explanation. But the evidence is clear: routine physical activity, sexual intercourse, and the normal stresses of daily life are not causes.
How Miscarriage Is Confirmed
A first-trimester miscarriage is typically confirmed through ultrasound. Doctors look for specific markers: if an embryo measures a certain size but shows no heartbeat, or if the gestational sac reaches 21 mm or larger without a visible embryo, the pregnancy is considered nonviable. When results are ambiguous on a first scan, a follow-up ultrasound seven or more days later can provide a definitive answer. If the sac was empty initially and still shows no embryo on the second scan, pregnancy loss is confirmed.
This careful, two-scan approach exists because early pregnancies can be difficult to date precisely. What looks like a failed pregnancy at five weeks may simply be a normal pregnancy that’s a few days earlier than expected. Doctors use conservative measurement thresholds specifically to avoid misdiagnosis.
Outlook After a Miscarriage
A single first-trimester miscarriage does not mean something is wrong with your ability to carry a pregnancy. Most people who miscarry once go on to have a healthy, full-term pregnancy without any special treatment. The 20% risk of miscarriage after one loss is only slightly higher than the baseline risk for any pregnancy. Doctors generally don’t recommend additional testing or intervention until someone has experienced two or three consecutive losses, at which point screening for conditions like antiphospholipid syndrome, thyroid dysfunction, or uterine abnormalities becomes more targeted.