Embryonic heart rate in human pregnancy

Seventy-two women with known gestational ages underwent serial human chorionic gonadotropin (hCG) measurements and transvaginal ultrasound studies with embryonic heart rate measurements. In 53 continuing singleton pregnancies, embryonic pulse appeared between days 26 and 32, was 80 beat per minute (bpm) on day 26, and increased linearly to plateau at 160-200 bpm by day 45 (r2 = 0.72). The pulse was always seen with hCG greater than 21,000 mIU/mL; pulse rate was correlated to embryonic crown-rump length. Of the remaining 19 pregnancies, 8 were anembryonic, 10 showed heart activity but subsequently aborted, and 1 was terminated. Absence of embryonic pulse by 32 days after conception or a serum hCG greater than 21,000 mIU/mL predicts spontaneous abortion; presence of a pulse may not guarantee successful continuation of the pregnancy since incidence of spontaneous abortion after visualization of embryonic pulse may be as high as 16%.