Journal of Ultrasound in Medicine, Vol 14, Issue 6 431-434, Copyright © 1995 by American Institute of Ultrasound in Medicine
Embryonic heart rate in the early first trimester: what rate is normal?
P. M. Doubilet and C. B. Benson
Department of Radiology, Harvard Medical School, Boston, Massachusetts, USA.
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.
