PURPOSE: To assess the outcome of early first-trimester pregnancies with slow embryonic heart rates. MATERIALS AND METHODS: Forty pregnant women had prenatal ultrasound scans obtained before 8 weeks gestation that demonstrated an embryonic heart rate of 90 beats per minute or less. Follow-up information was used to determine pregnancy status at the end of the first trimester. RESULTS: Thirty-two embryos died before the end of the first trimester, five survived the first trimester, and three were lost to follow-up. Fetal demise occurred in all seven embryos with heart rates of less than 70 beats per minute, 10 of 11 with heart rates of 70-79 beats per minute, and 15 of 19 with heart rates of 80-90 beats per minute. All 32 pregnancy losses occurred within 10.5 weeks gestation. CONCLUSION: An embryonic heart rate of 90 beats per minute or less early in the first trimester carries a dismal prognosis, with a very high likelihood of fetal demise before the end of the first trimester. Demise occurred in all embryos with heart rates less than 70 beats per minute.