In a prospective study of 500 patients who had an ultrasonically viable pregnancy at less than 12 weeks' gestation, the spontaneous abortion rate was 2% overall. In those women with a history of spontaneous abortion, the abortion rate increased tenfold. Spontaneous abortion at less than ten weeks' gestation was up to three times higher than that at greater than ten weeks' gestation; this may have implications when deciding on the timing of first-trimester diagnostic procedures.