Why Miscarriage Happens: Causes and What They Mean for You

What actually causes most miscarriages?

Chromosomal abnormalities: This is the most common identifiable cause of early pregnancy loss. Studies using conventional cytogenetics find chromosomal abnormalities in 50–60% or more of first-trimester miscarriages 3Ref 3Pylyp LY, Spynenko LO, Verhoglyad NV, Mishenko AO, Mykytenko DO, Zukin VD (2018).Chromosomal abnormalities in products of conception of first-trimester miscarriages detected by conventional cytogenetic analysis: a review of 1000 cases.Chromosomal abnormalities identified in approximately 50-60% of first-trimester miscarriages; rate increases with maternal age; most common abnormalities are numerical (aneuploidies). During egg or sperm formation, or in the first cell divisions after fertilization, a chromosome may be lost, duplicated, or rearranged. The result is an embryo that cannot develop normally. This process happens at random — it is not inherited, and it is not caused by activity, diet, stress, or anything the pregnant person did or did not do.

The risk of chromosomal errors increases with the age of the egg, which is why miscarriage rates rise with maternal age.

Uterine structural problems: A small number of losses are attributed to physical features of the uterus — a septum dividing the cavity, fibroids pressing into the cavity, or adhesions (scarring) that interfere with implantation 1Ref 1Practice Committee of the American Society for Reproductive Medicine (2012).Evaluation and treatment of recurrent pregnancy loss: a committee opinion.Standard workup for recurrent pregnancy loss including antiphospholipid antibody testing, karyotype, uterine evaluation, and thyroid assessment; antiphospholipid syndrome as the most clearly established treatable cause; genetic testing of pregnancy tissue.

Hormonal factors: Uncontrolled thyroid disease and poorly controlled diabetes can increase miscarriage risk. Progesterone deficiency is often discussed, though the evidence for treating luteal phase deficiency specifically is mixed.

Antiphospholipid syndrome: An autoimmune condition in which the immune system produces antibodies that interfere with normal blood clotting. It is the most clearly established treatable cause of recurrent pregnancy loss 1Ref 1Practice Committee of the American Society for Reproductive Medicine (2012).Evaluation and treatment of recurrent pregnancy loss: a committee opinion.Standard workup for recurrent pregnancy loss including antiphospholipid antibody testing, karyotype, uterine evaluation, and thyroid assessment; antiphospholipid syndrome as the most clearly established treatable cause; genetic testing of pregnancy tissue2Ref 2American College of Obstetricians and Gynecologists (2018).ACOG Practice Bulletin No. 200: Early Pregnancy Loss.Definition and prevalence of early pregnancy loss; no evidence that exercise, sexual activity, stress, or minor physical events cause miscarriage; expectant management and prognosis after single miscarriage. Testing for it is a standard part of the recurrent pregnancy loss evaluation.

Genetic causes in the parents: Balanced chromosomal translocations — where a segment of one chromosome has swapped position with another, without gaining or losing genetic material — can lead to repeated chromosomal errors in embryos. This is found in a minority of couples with recurrent loss 1Ref 1Practice Committee of the American Society for Reproductive Medicine (2012).Evaluation and treatment of recurrent pregnancy loss: a committee opinion.Standard workup for recurrent pregnancy loss including antiphospholipid antibody testing, karyotype, uterine evaluation, and thyroid assessment; antiphospholipid syndrome as the most clearly established treatable cause; genetic testing of pregnancy tissue.

What does NOT cause miscarriage?

This is important to say plainly: miscarriage is not caused by:

• Exercise or physical activity during pregnancy
• Emotional stress or a frightening experience
• Sex during pregnancy
• Minor falls or routine bumps
• Eating most foods (barring known unsafe foods in pregnancy)
• Working a standard job

The guilt and self-questioning that often follow a pregnancy loss are understandable and widely experienced — but they are not medically supported in the vast majority of cases 2Ref 2American College of Obstetricians and Gynecologists (2018).ACOG Practice Bulletin No. 200: Early Pregnancy Loss.Definition and prevalence of early pregnancy loss; no evidence that exercise, sexual activity, stress, or minor physical events cause miscarriage; expectant management and prognosis after single miscarriage. The loss is almost always due to events in the embryo's chromosomes that were outside anyone's control.

When does evaluation make sense, and what does it involve?

For a single miscarriage in someone without known risk factors, investigation is not standard practice — the most likely cause is a random chromosomal event in that pregnancy, and the chance of a subsequent successful pregnancy is high 1Ref 1Practice Committee of the American Society for Reproductive Medicine (2012).Evaluation and treatment of recurrent pregnancy loss: a committee opinion.Standard workup for recurrent pregnancy loss including antiphospholipid antibody testing, karyotype, uterine evaluation, and thyroid assessment; antiphospholipid syndrome as the most clearly established treatable cause; genetic testing of pregnancy tissue2Ref 2American College of Obstetricians and Gynecologists (2018).ACOG Practice Bulletin No. 200: Early Pregnancy Loss.Definition and prevalence of early pregnancy loss; no evidence that exercise, sexual activity, stress, or minor physical events cause miscarriage; expectant management and prognosis after single miscarriage.

For recurrent pregnancy loss — two or more pregnancy losses — evaluation is recommended 1Ref 1Practice Committee of the American Society for Reproductive Medicine (2012).Evaluation and treatment of recurrent pregnancy loss: a committee opinion.Standard workup for recurrent pregnancy loss including antiphospholipid antibody testing, karyotype, uterine evaluation, and thyroid assessment; antiphospholipid syndrome as the most clearly established treatable cause; genetic testing of pregnancy tissue. A standard workup typically includes:

• Chromosome testing of both partners (karyotype) — checks for balanced translocations
• Antiphospholipid antibody testing — rules out or confirms antiphospholipid syndrome
• Uterine evaluation — ultrasound or sonohysterogram to assess the uterine cavity
• Thyroid function and prolactin — hormonal contributors

Genetic testing of the pregnancy tissue (products of conception) when available can provide direct information about whether a chromosomal error caused a specific loss. ASRM now recommends offering genetic evaluation of miscarriage tissue to all patients with pregnancy loss, as it can inform prognosis 1Ref 1Practice Committee of the American Society for Reproductive Medicine (2012).Evaluation and treatment of recurrent pregnancy loss: a committee opinion.Standard workup for recurrent pregnancy loss including antiphospholipid antibody testing, karyotype, uterine evaluation, and thyroid assessment; antiphospholipid syndrome as the most clearly established treatable cause; genetic testing of pregnancy tissue.

Evaluation for recurrent pregnancy loss is typically coordinated by a reproductive endocrinologist or maternal-fetal medicine specialist.

What are the chances of a successful pregnancy after a miscarriage?

After a single miscarriage, the likelihood of a subsequent successful pregnancy is high for most people, particularly those under 35 2Ref 2American College of Obstetricians and Gynecologists (2018).ACOG Practice Bulletin No. 200: Early Pregnancy Loss.Definition and prevalence of early pregnancy loss; no evidence that exercise, sexual activity, stress, or minor physical events cause miscarriage; expectant management and prognosis after single miscarriage. The prognosis is affected by:

• Age: Egg quality declines with age, increasing both the risk of chromosomal abnormalities and the risk of miscarriage 3Ref 3Pylyp LY, Spynenko LO, Verhoglyad NV, Mishenko AO, Mykytenko DO, Zukin VD (2018).Chromosomal abnormalities in products of conception of first-trimester miscarriages detected by conventional cytogenetic analysis: a review of 1000 cases.Chromosomal abnormalities identified in approximately 50-60% of first-trimester miscarriages; rate increases with maternal age; most common abnormalities are numerical (aneuploidies)
• Number of prior losses: Each additional loss does incrementally increase the risk of another, which is why evaluation after two losses is meaningful
• Identified cause: When antiphospholipid syndrome is found and treated with low-dose aspirin plus low-molecular-weight heparin, live birth rates improve substantially 1Ref 1Practice Committee of the American Society for Reproductive Medicine (2012).Evaluation and treatment of recurrent pregnancy loss: a committee opinion.Standard workup for recurrent pregnancy loss including antiphospholipid antibody testing, karyotype, uterine evaluation, and thyroid assessment; antiphospholipid syndrome as the most clearly established treatable cause; genetic testing of pregnancy tissue

For couples with unexplained recurrent pregnancy loss, supportive care — including progesterone supplementation in some protocols — is sometimes offered, though the evidence base continues to evolve. A reproductive endocrinologist can give you the most personalized picture.

What about the emotional experience?

Pregnancy loss is a genuine grief experience. There is no standard timeline for how long it should take to feel better, and feelings of sadness, anxiety, anger, or disconnection are all common. Partners may grieve differently and on different timelines.

Support resources — including mental health clinicians familiar with pregnancy loss and support groups — are available. Bringing up how you are doing emotionally at your next clinical appointment is appropriate; your care team can connect you with support.