Reference intervals for first trimester embryonic/fetal heart rate in a Thai population.

Objective: To establish reference intervals for first trimester embryonic/fetal heart rate in a Thai population.
Methods: A prospective descriptive cross-sectional study was performed on normal pregnant women registered at the antenatal clinic in their first trimester. All had a good menstrual history and the calculated gestational age using crown-rump length (CRL) in the first trimester ultrasound was in agreement. Crown-rump length and embryonic/fetal heart rate measurements were obtained by a 5 MHz vaginal probe combined with a duplex color Doppler machine. All of the newborns were proved to be normal at birth. We constructed and compared embryonic/fetal heart rate expressed as beats/min relating to gestational age (GA) (days) and CRL. The data was analyzed and the best fit mathematical model was derived using the SPSS computer program.
Results: 547 cases were enrolled into the study. The median embryonic/fetal heart rate increased from 124 beats/min at GA of 40-44 days to 177 beats/min at GA 60-64 days, thereafter, embryonic/fetal heart rate gradually decreased to 159 beats/min at GA 95-99 days. The best fit regression equation was: embryonic/fetal heart rate (beats/min) = -21.666 + 5.4796 GA (day) - 0.0383 GA (day)2, R = 0.5, p < 0.0001). In addition, embryonic/fetal heart rate also correlated with CRL. The median embryonic/fetal heart rate increased from 131 beats/min at CRL of 5-9 mm to 177 beats/min at CRL 20-34 mm, thereafter, embryonic/fetal heart rate gradually decreased to 155 beats/min at CRL 75-79 mm. The best fit regression equation was: embryonic/fetal heart rate (beats/min) = 127.361 + 3.4939 CRL (mm) - 0.0749 CRL (mm)2 + 0.0004 CRL (mm)3, R = 0.58, p < 0.0001).
Conclusion: Reference intervals for first trimester embryonic/fetal heart rate in a Thai population were established. Our reference intervals may be useful for further studies, such as the prediction of spontaneous abortion or chromosomal disorders after ultrasound-proven viability in the first trimester.