What Causes Miscarriage? A Plain-Language Medical Guide

Most first-trimester miscarriages — roughly 50–70% — are caused by chromosomal abnormalities in the embryo. These are random genetic errors, not anything the pregnant person caused. Other causes include uterine structural issues, immune conditions, hormonal factors, and (rarely) infection. In roughly 50% of recurrent losses, no cause is ever identified.

After a miscarriage, "why did this happen?" is the first and most urgent question. The medical answer — usually some variation of "chromosomal abnormality" — can feel both informative and deeply unsatisfying at the same time.

This guide covers all known causes of pregnancy loss, organized by how common they are and what they mean for future pregnancies.

The Most Common Cause: Chromosomal Abnormalities

The majority of first-trimester losses are caused by chromosomal abnormalities in the embryo — trisomies (extra chromosomes), monosomies (missing chromosomes), and structural rearrangements that prevent normal development.

These errors arise primarily during egg formation (meiosis) and become more frequent with maternal age. A 28-year-old produces roughly 20–25% chromosomally abnormal eggs; a 42-year-old produces roughly 70–80% abnormal eggs. This is why miscarriage risk rises so steeply with age.

These errors are random. They are not caused by:

• Physical activity, including exercise, sex, or lifting
• Stress or emotional state
• Prior contraceptive use
• Prior terminations
• Diet or nutrition (unless severe deficiency)
• Environmental exposures at typical background levels

A pregnancy lost to chromosomal abnormality is not a sign that something is wrong with the reproductive system or that future pregnancies will also be affected. Each new embryo is a new genetic combination.

Our detailed guide on chromosomal abnormalities and miscarriage covers the biology in more depth.

Uterine Structural Issues (~10–15% of Recurrent Losses)

Several anatomical variations can affect the uterine environment and increase miscarriage risk:

Uterine septum: A band of fibrous tissue dividing the uterine cavity. It can impair blood flow to an implanting embryo. Correction by hysteroscopic resection is associated with improved outcomes, though evidence from randomized controlled trials is limited.

Submucosal fibroids: Benign muscle tumors that protrude into the uterine cavity can interfere with implantation. Hysteroscopic removal is associated with improved fertility outcomes.

Asherman's syndrome: Intrauterine adhesions (scar tissue), often following uterine surgery, can reduce the surface area available for implantation.

Bicornuate or arcuate uterus: Shape variations of the uterus. The clinical significance is variable; many women with these variants have normal pregnancies, but some structural shapes are associated with second-trimester loss risk.

Antiphospholipid Syndrome (APS): The Most Important Treatable Cause

Antiphospholipid syndrome (APS) is an autoimmune condition affecting roughly 5–15% of women with recurrent pregnancy loss. The immune system produces antibodies (anticardiolipin, lupus anticoagulant, anti-beta2 glycoprotein I) that interfere with phospholipid-binding proteins, disrupting placental function and promoting thrombosis.

APS is significant because it is treatable. Low-dose aspirin combined with low-molecular-weight heparin (LMWH) during pregnancy reduces the miscarriage rate in women with APS from roughly 70–90% (without treatment) to around 25–30%. This is one of the most dramatic treatment effects seen in RPL management.

Testing requires two positive antibody results at least 12 weeks apart. A single positive result is insufficient for diagnosis because false positives are common.

Thyroid and Hormonal Factors

Hypothyroidism: Thyroid hormone affects endometrial development and implantation. Both overt hypothyroidism (elevated TSH, low free T4) and subclinical hypothyroidism (TSH 2.5–10 mIU/L with normal T4) have been associated with increased miscarriage risk. Most specialists target a TSH below 2.5 mIU/L in women with RPL.

Hyperthyroidism: Poorly controlled hyperthyroidism is also associated with adverse pregnancy outcomes. Careful management with the lowest effective dose of antithyroid medication is standard.

Progesterone: Progesterone supports the uterine lining (endometrium) after ovulation, preparing it for implantation and maintaining early pregnancy. Luteal phase deficiency — insufficient progesterone in the second half of the cycle — has been proposed as a cause of early loss, though the evidence is contested. Progesterone supplementation is often offered empirically in women with prior losses.

Uncontrolled diabetes: Women with uncontrolled preconception and early-pregnancy diabetes (high HbA1c) have elevated miscarriage rates. This is largely reversible with good glycemic control.

Inherited Thrombophilias: A Nuanced Picture

Inherited thrombophilias — genetic variants that increase blood clotting tendency — have been extensively studied as potential RPL causes. The most studied variants are factor V Leiden, prothrombin G20210A mutation, and MTHFR variants.

The evidence for their role in early miscarriage is weak and has been downgraded in recent guidelines. Most major guidelines (ACOG, ESHRE, RCOG) no longer recommend routine thrombophilia testing for women with first-trimester RPL. The exception is testing for antiphospholipid syndrome, which is an acquired (not inherited) thrombophilia and remains firmly in the standard workup.

Second-trimester losses and late pregnancy complications (severe pre-eclampsia, fetal growth restriction) have a stronger association with inherited thrombophilias than first-trimester miscarriage.

Rare Causes

Uterine or cervical infection: Certain infections — bacterial vaginosis, chlamydia, Listeria, certain bacterial infections — can cause miscarriage when severe or untreated, but are rare causes of first-trimester loss in women receiving prenatal care.

Cervical insufficiency: More relevant to second-trimester loss. The cervix begins to dilate without uterine contractions, leading to a pregnancy that is delivered too early. Signs include painless dilation in the second trimester. Treatment is cervical cerclage placement.

Toxin exposure at high doses: Certain occupational exposures (radiation above specific thresholds, some solvents) are associated with miscarriage. Everyday exposures, including coffee in moderate amounts, are not.

What "Unexplained" RPL Means

Roughly 50% of RPL cases have no identifiable cause after complete workup. This is genuinely frustrating to hear. The honest explanation is that our tools for identifying miscarriage causes are imperfect, and some causes are not yet understood.

The prognosis for unexplained RPL is still meaningful: 60–75% of couples with unexplained RPL achieve a live birth in a subsequent pregnancy without any specific treatment. Empirical progesterone is often offered and may provide modest benefit.

Use our miscarriage risk calculator to understand how your personal risk factors combine in a current pregnancy. For a focus on the RPL diagnostic workup, see our guide on recurrent miscarriage causes and testing.

Sources: ACOG Practice Bulletin No. 200: Early Pregnancy Loss (2018). Regan L et al. (1989). Prospective study of spontaneous abortion. BMJ. Rai R, Regan L. (2006). Recurrent miscarriage. Lancet. Brosens I et al. (2011). Uterine junctional zone. Human Reproduction Update.