To discover whether first-trimester spontaneous abortion can be predicted by embryonic heart rate (EHR), we performed a cross-sectional study during the first trimester of pregnancy using high-frequency transvaginal sonography combined with pulsed Doppler. Heart rate was measured in 603 embryos; of these, 580 continued beyond 13 weeks' gestation and 23 ended in first-trimester spontaneous abortion. Based on the continuing pregnancies, we constructed and compared EHR nomograms relating to gestational age, mean diameter of the gestational sac, and crown-rump length (CRL). Embryonic heart rate correlated best with CRL (r = 0.87), and the correlation was best described by a second-degree polynomial regression equation. The mean EHR increased progressively from 110 beats per minute (bpm) at CRL of 3-4 mm to 171-178 bpm at CRL 15-32 mm. At CRL greater than 32 mm, the EHR remained stable at a mean of 170 bpm. The EHRs of the 23 embryos that spontaneously aborted in the first trimester were evaluated according to these nomograms. In 15 cases, the EHR fell outside the 95% confidence interval for CRL (sensitivity 65%), but it was within normal limits in eight (false-negative rate 35%). In ten embryos for which pregnancy continued beyond 13 weeks, the EHR fell outside the 95% confidence interval (specificity 98%, false-positive rate 2%). Our findings suggest that EHR measurements in early pregnancy may be useful in the prediction of first-trimester spontaneous abortion after ultrasound-proven viability.