Objective: We conducted a study to determine the upper limits of normal embryonic heart rate at or before 7.0 weeks' gestational age and to assess outcome of pregnancies in which the early embryonic heart rate is rapid.
Subjects and methods: We recorded embryonic heart rates in 2817 sonograms at or before 7.0 weeks' gestation performed between January 1993 and June 1998. The upper limit of normal heart rate in two gestational age ranges (before 6.3 weeks and 6.3-7.0 weeks) was computed as the average of two values: mean heart rate + 1.96 standard deviations and the rate above which 2.5% of embryos in our population were measured. Pregnancy outcome in cases with rapid embryonic heart rates was compared with pregnancy outcome in a control group with normal rates.
Results: The upper limit of normal heart rate was 134 beats per minute before 6.3 weeks' gestation and 154 beats per minute at 6.3-7.0 weeks' gestation. Forty-one embryos had rapid early heart rates and known first-trimester outcome, of which 37 (90.2%) were alive at the end of the first trimester. Pregnancy outcome was available in 33 of the 37 first-trimester survivors (four were lost to follow-up before delivery), and 30 of these 33 (90.9%) were healthy neonates. These short- and long-term outcomes were not significantly different from those of the control group of embryos with normal early heart rates (p > 0.20, Fisher's exact test).
Conclusion: A rapid early embryonic heart rate is one that is at least 135 beats per minute before 6.3 weeks or at least 155 beats per minute at 6.3-7.0 weeks. Pregnancies in which the embryo has a rapid early heart rate have a good prognosis, with a high likelihood of normal outcome.