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Embryonic Heart Rate and Age

Austin Radiological Association and Austin Community College, Department of Diagnostic Medical Sonography; Austin Radiological Association, Bailey Square Medical-Surgical Building #102, 1111 West 34th Street, Austin, TX 78705.

James A. Cunyus, MD

Austin Radiological Association and Austin Community College, Austin, Texas.

Laurence F. Johnson, MBA, BA

Parallel Studies Division, Austin, Texas.

This study used linear regression analysis to show that embryonic heart rate (EHR) can be used in the clinical setting to provide an estimate of embryonic age that correlates well with the fetal crown-rump length age (CRL; r = 0.856). A regression was performed on data from 275 examinations in which an embryonic heart rate was recorded before the ninth menstrual week from a cross-sectional population of 1,136 examinations throughout pregnancy. In this analysis, EHR was used as the independent variable and CRL as the dependent variable to yield the following formula for CRL age in menstrual days: CRL Age (menstrual days) = 8.18 + 0.285 EHR. Based on this result, a rounded version of the formula was sought that would provide essentially the same estimated fetal age as the regression formula for the range of EHR that occurred in the sample (84-193 beats/min) yet allow for a simpler computation. Within that range of EHR, the following clinical versions of the regression formula were found to produce results that did not differ significantly from the original formula: Days of Gestation = 0.3 EHR -8 and Menstrual Days Age = 0.3 EHR + 6. Both the original regression and the clinical versions yielded 95% prediction intervals of approximately ± 8 days across the range of EHR that occurred in the sample. While fetal age based on this method is only an approximation of the CRL age, the heart rate can be obtained using time/motion mode sonography when only the less accurate gestational sac diameters are available. The EHR is useful to corroborate early age estimates, especially when using transvaginal transducers and may provide useful information concerning cardiac development.

Key Words: embryo • embryonic heart rate • fetal heart rate • fetal age • sonography • gestational age • menstrual age

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