Blood samples were obtained from 5 women during the menstrual cycle in which they had conceived. Although neither the women nor their physicians were clinically aware that conception had occurred, one or more of the blood specimens from each woman contained human chorionic gonadotropin (hCG), as measured by a specific radioimmunoassay. Each patient underwent a spontaneous abortion with 4 of the 5 women describing a delayed, but otherwise normal, menses. The fifth noted an unusually abundant amount of vaginal bleeding. Immunologic urinary pregnancy slide tests were negative in the 4 women on whom the tests were performed. The levels of hCG in the serum of these women were abnormally low for the estimated length of gestation, suggesting that defective trophoblastic function may be important in the pathogenesis of this phenomenon. These examples illustrate that some women may conceive and abort so early that the pregnancy and the abortion may remain subclinical. Therefore, it is likely that the true incidence of spontaneous abortions during the first trimester is greater than the presently accepted incidence of 15%.