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Spontaneous abortion: Risk factors, etiology, clinical manifestations, and diagnostic evaluation

Togas Tulandi, MD, MHCM
Haya M Al-Fozan, MD
Section Editors
Deborah Levine, MD
Robert L Barbieri, MD
Deputy Editor
Kristen Eckler, MD, FACOG


Spontaneous abortion, or miscarriage, is defined as a clinically recognized pregnancy loss before the 20th week of gestation [1,2]. The World Health Organization (WHO) defines it as expulsion or extraction of an embryo or fetus weighing 500 g or less. The term "fetus" will be used throughout this discussion, although the term "embryo" is the correct developmental term at ≤10 weeks of gestation.

The etiology, risk factors, and diagnostic issues relating to spontaneous abortion are reviewed here. Recurrent abortion and management issues are discussed separately. (See "Evaluation of couples with recurrent pregnancy loss" and "Spontaneous abortion: Management".)


Intrauterine pregnancy of uncertain viability – Transvaginal ultrasonography shows an intrauterine gestational sac with no embryonic heartbeat (and no findings of definite pregnancy failure) [3].

Pregnancy of unknown location – A urine or serum pregnancy test is positive and there is no intrauterine or ectopic pregnancy seen on transvaginal ultrasonography.


Spontaneous abortion is the most common complication of early pregnancy [1]. The frequency decreases with increasing gestational age. The incidence of spontaneous abortion (miscarriage) in clinically recognized pregnancies up to 20 gestational weeks is 8 to 20 percent. However, the incidence among women who have previously had a child is much lower (5 percent) [4,5]. The overall risk of spontaneous abortion after 15 weeks is low (about 0.6 percent) for chromosomally and structurally normal fetuses, but varies according to maternal age and ethnicity [6].


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