Embryonic heart rates below 90 beats per minute at 6 to 8 weeks of gestation have been shown to be associated with a high likelihood of subsequent first trimester demise. The boundary between slow and normal heart rates has not been established, however, in that previous studies do not identify the heart rate for each gestational age at which prognosis plateaus. We evaluated sonograms of singleton pregnancies at 6 to 8 weeks to determine the relationship between heart rate and first trimester outcome in four gestational age subgroups: < or = 6.2, 6.3-7.0, 7.1-7.5, and 7.6-8.0 weeks. Below 6.2 weeks, prognosis improved as heart rate increased to 100 beats per minute, then plateaued for rates > or = 100 beats per minute. At 6.3-7.0 weeks, prognosis improved up to 120 beats per minute, then plateaued. Heart rate norms could not be established at 7.1-8.0 weeks because of the small number of embryonic or fetal deaths in this age group; however, all embryos with heart rates below 110 beats per minute at 7.1-8.0 weeks died. We conclude that the lower limit of normal is 100 beats per minute up to 6.2 weeks' gestation and 120 bpm at 6.3-7.0 weeks.