OBJECTIVE To examine the frequency of pregnancy loss following successful development of anatomical embryonic landmarks identified with endovaginal ultrasound. METHODS Two hundred thirty-two women with positive urinary pregnancy tests and no antecedent history of vaginal bleeding had endovaginal sonography performed at the initial visit and at subsequent visits as indicated clinically. The presence of anatomical and embryonic structures (gestational sac, yolk sac, embryo) and cardiac activity was recorded. Patients were followed until delivery unless sonographic evidence of nonviability was seen or spontaneous loss occurred. RESULTS Twenty-seven losses occurred during the embryonic period, four losses occurred in the fetal period, and there were 201 live births. If a gestational sac developed, subsequent loss of viability in the embryonic period occurred in 11.5%; loss rates were 8.5% with a yolk sac, 7.2% for an embryo up to 5 mm, 3.3% for an embryo of 6-10 mm, and 0.5% for an embryo larger than 10 mm. No pregnancies were lost between 8.5 and 14 menstrual weeks. The fetal loss rate after 14 weeks was 2.0%. CONCLUSIONS The rate of early pregnancy loss decreases successively with gestational age and is virtually complete by the end of the embryonic period (70 days after onset of the last menstrual period). Subsequent pregnancy losses in the fetal period occur between 14 and 20 weeks. This pattern of early pregnancy death suggests a period of embryonic loss distinct from one of fetal loss. Based on these data, the physiologic significance of the traditional boundary of the first trimester as an appropriate dividing time line for early pregnancy may be questioned.