Purpose. Heart rates in embryos (6-8 weeks' gestation) have rarely been studied. We attempted to identify embryonic heart rates at 4 stages during 6-8 weeks' gestation and to relate our findings to fetal outcome at the end of the first trimester. We also tried to determine the lower limit of the heart rate associated with a favorable outcome for each group.
Methods. We prospectively studied 2,164 singleton pregnancies between 1992 and 1995. Gestational age was calculated from the known date of the beginning of the last menstrual period and was confirmed by sonographic measurement of the crown-rump length. M-mode sonography was used to calculate the embryonic heart rate as the mean of at least 3 waves in beats/minute.
Results. Mean embryonic heart rates (ยฑ standard deviation) were 111 ยฑ 14 beats/min for group 1 (42-45 days), 125 ยฑ 15 beats/min for group 2 (46-49 days), 145 ยฑ 14 beats/min for group 3 (50-52 days), and 157 ยฑ 13 beats/min for group 4 (53-56 days). Statistically significant differences (p < 0.01) in the mean heart rates were seen between all groups. A heart rate between 116 and 125 beats/min was associated with the fewest losses in groups 1 (10.8% losses) and 2 (8.6%). In groups 3 and 4, a heart rate of 146 beats/min or more was associated with the fewest losses (5.5% and 3.3%, respectively). A highly significant (p < 0.01) linear decrease was seen in fetal losses with increasing gestational age. Embryos with heart rates below 85 beats/ min during 6-8 weeks' gestation did not survive.
Conclusions. The heart rate during 6-8 weeks' gestation is associated with fetal outcome at the end of the first trimester and identifies those embryos that are in jeopardy.