Chromosomal abnormalities in the embryo are the main cause of missed miscarriage, accounting for roughly two-thirds of cases. A missed miscarriage, sometimes called a silent miscarriage, happens when an embryo stops developing but the body doesn’t recognize the loss or begin to pass the pregnancy tissue. Many people only find out during a routine ultrasound, which makes the diagnosis especially shocking.
Chromosomal Abnormalities Drive Most Cases
When researchers analyze tissue from missed miscarriages, about 67% of cases show abnormal chromosomes in the embryo. These aren’t inherited genetic conditions in most cases. They’re random errors that happen when the egg and sperm combine, or during the earliest cell divisions afterward. The embryo ends up with too many or too few chromosomes, and it simply can’t develop normally.
The most common specific abnormality is trisomy 16, where the embryo has three copies of chromosome 16 instead of two. This single error accounts for about 27% of all chromosomally abnormal missed miscarriages. The next most common is Turner syndrome (a missing X chromosome), making up about 18% of cases. Triploidy, where the embryo has an entire extra set of chromosomes (69 instead of 46), accounts for roughly 7%. These errors are essentially a biological lottery. There’s nothing a person could have done differently to prevent them.
How Maternal Age Affects Risk
Age is the strongest known risk factor for miscarriage because older eggs are more likely to produce chromosomal errors during cell division. A large Norwegian study tracking over 237,000 pregnancies found the risk of miscarriage was lowest for women aged 25 to 29, at about 10%. By age 35 to 39, it rose to nearly 17%. At 40 to 44, it jumped to 32%. And for women over 45, more than half of recognized pregnancies ended in miscarriage.
Younger women aren’t immune, though. Women under 20 had a miscarriage rate of about 16%, slightly higher than those in their late twenties. The overall baseline across all ages: about 15% of pregnancies recognized after a missed period end in first-trimester miscarriage.
Hormonal Factors, Especially Progesterone
After chromosomal problems, hormonal imbalances are among the most studied contributors. Progesterone plays a critical role in the earliest weeks of pregnancy. It helps the fertilized egg implant in the uterine wall, supports the developing placenta, calms uterine contractions, and helps the immune system tolerate the pregnancy rather than attacking it as foreign tissue.
When progesterone levels drop too low during early pregnancy, the environment the embryo needs to survive can fall apart. A 2024 scoping review found consistent evidence linking progesterone deficiency to first-trimester miscarriage, with seven studies identifying it as a strong standalone predictor of early pregnancy loss. This deficiency can stem from a luteal phase defect, where the ovary doesn’t produce enough progesterone after ovulation, or from problems with the early placenta failing to take over hormone production on schedule.
Blood Clotting Disorders and Placental Damage
Antiphospholipid syndrome (APS) is an autoimmune condition where the body produces antibodies that make blood more likely to clot. During pregnancy, these abnormal clots can form in the tiny blood vessels of the placenta. The result is a placenta that can’t deliver enough oxygen and nutrients to the embryo.
The damage goes beyond simple clotting. These antibodies also strip away a protective anticoagulant protein that normally coats the placental surface, and there’s laboratory evidence they can directly interfere with the cells that form the placenta in the first place. Because the placenta fails gradually and silently, APS is particularly associated with missed miscarriage rather than the kind where bleeding serves as an early warning sign. APS is also linked to later pregnancy complications and recurrent loss, so it’s one of the conditions doctors specifically test for after repeated miscarriages.
Uterine Shape and Structural Problems
The physical shape of the uterus matters more than many people realize. About 7% of women have some form of uterine malformation, and certain types significantly increase miscarriage risk. A septate uterus, where a wall of tissue divides the uterine cavity, is one of the most problematic. Women with a septate, bicornuate (heart-shaped), or unicornuate (half-sized) uterus had miscarriage rates of about 19% in one study, compared to 8% in women with a normally shaped uterus.
The mechanism is straightforward: an abnormally shaped cavity can limit blood supply to the area where the embryo implants, or simply not provide enough room for normal early growth. A complete septate uterus was associated with both higher miscarriage rates and more serious bleeding complications. Many women don’t know they have a uterine malformation until they experience pregnancy loss and undergo imaging.
Other Contributing Factors
Several additional conditions appear in the research as contributing causes, though each is less common than chromosomal errors. Uncontrolled thyroid disease, particularly an underactive thyroid, can disrupt the hormonal environment needed to sustain early pregnancy. Poorly managed diabetes raises miscarriage risk through effects on blood vessel development and inflammation. Certain infections during early pregnancy can interfere with embryo development without causing obvious symptoms.
Inherited thrombophilia, a genetic tendency toward excessive blood clotting that’s separate from APS, can cause similar placental damage. Environmental exposures, including certain chemicals and high levels of stress hormones, have also been identified as potential contributors, though their individual impact is harder to quantify.
How a Missed Miscarriage Is Confirmed
Because a missed miscarriage produces no bleeding or cramping in many cases, it’s typically discovered on ultrasound. Doctors use specific measurements to make the diagnosis and avoid any chance of error. If the embryo measures longer than 7 mm from head to rump and has no heartbeat, that confirms a missed miscarriage. If a gestational sac measures larger than 25 mm across and contains no visible embryo or yolk sac, that’s classified as an empty sac miscarriage, a subtype of missed miscarriage where the embryo never formed.
When measurements fall below those cutoffs, doctors will schedule a follow-up ultrasound at least one week later. If there’s still no heartbeat or the sac remains empty at that point, the diagnosis is confirmed. This waiting period exists specifically to prevent misdiagnosing a very early but viable pregnancy.
What Happens After Diagnosis
Once a missed miscarriage is confirmed, there are three main paths forward. Expectant management means waiting for the body to recognize the loss and pass the tissue on its own. Medical management uses medication to prompt the uterus to empty. Surgical management involves a brief procedure to remove the pregnancy tissue.
A Cochrane review of 59 trials covering over 12,500 women found that both surgical and medical approaches were significantly more effective at completing the miscarriage than waiting. Surgical methods ranked highest overall, followed by medical options. Expectant management had the lowest success rate and the highest chance of needing unplanned or emergency surgery. Notably, the advantage of active treatment was even more pronounced for missed miscarriages specifically, compared to incomplete miscarriages where the process has already started naturally. This makes sense: in a missed miscarriage, the body hasn’t yet begun responding to the loss, so it may need more help to do so.
The choice between these options depends on how far along the pregnancy was, personal preference, and medical circumstances. Many people choose medical or surgical management simply because the uncertainty of waiting, sometimes for weeks, adds emotional strain to an already difficult situation.