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Spontaneous abortion

  • Lesion

Most early miscarriages happen because the embryo has the wrong number of chromosomes; a smaller share comes from the uterus, hormones, clotting or infection.

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What it is

Spontaneous abortion is loss of a pregnancy before the fetus can survive, and it ends a substantial share of recognised pregnancies, most in the first trimester. About half of first-trimester losses are caused by chromosome abnormalities of the embryo, mainly trisomies, monosomy X and triploidy. Maternal causes include uterine malformations, submucosal leiomyomas, poorly controlled endocrine disease, antiphospholipid syndrome and other thrombophilias, infections and toxins. Examining the products of conception confirms an intrauterine pregnancy and can reveal a molar pregnancy or clues to the cause.

Structure

  • Normal-looking villi with decidua and fibrin are common in losses from any cause.
  • Hydropic villi with irregular outlines and trophoblast inclusions suggest an abnormal karyotype or a partial mole; circumferential trophoblast proliferation suggests a complete mole.
  • Decidual arteries with fibrinoid necrosis and thrombosis point to a maternal vascular cause such as antiphospholipid syndrome.
  • Neutrophils in the membranes or villi indicate infection (septic abortion).

Step by step

  1. An error in meiosis, usually maternal and more frequent with older maternal age, gives the embryo an abnormal chromosome number.
  2. The embryo develops abnormally and the placenta forms poorly.
  3. Alternatively, the uterus, hormones or maternal blood vessels cannot support an otherwise normal embryo.
  4. The embryo dies, the decidua separates and bleeding and cramps expel the products of conception.

Clinical notes

  • Presents with bleeding and cramping in early pregnancy; ultrasound and hCG distinguish threatened, incomplete, complete and missed abortion.
  • Recurrent pregnancy loss warrants a search for parental chromosome rearrangements, uterine anomalies and antiphospholipid antibodies.
  • Retained tissue can become infected (septic abortion) or bleed, and repeated curettage can cause intrauterine adhesions.

Test yourself

Think of the answer, then tap the question to see it.

What is the most common cause of first-trimester spontaneous abortion?

Chromosomal abnormality of the embryo, mainly autosomal trisomies, followed by monosomy X and triploidy.

What does finding chorionic villi in a uterine specimen prove?

That the pregnancy was intrauterine, which effectively excludes an ectopic pregnancy except in the rare combined case.

Which acquired autoimmune disorder causes recurrent pregnancy loss with placental thrombosis?

Antiphospholipid syndrome.

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Early Pregnancy Loss (Spontaneous Abortion) (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.