Using simultaneous B-mode and time-motion transvaginal ultrasonography we measured embryonic heart rate (EHR) in dated human embryos during the first trimester in a prospective study. In 160 pregnant women who had undergone assisted reproductive procedures, 139 singleton embryos and 46 embryos from 21 multiple pregnancies were studied between the 37th and 98th postmenstrual day. Because multiple measurements in one embryo at different ages were possible, a total of 348 measurements were taken. EHR in dated human embryos shows an increase up to 63 postmenstrual days or 22 mm greatest length. Thereafter a steady decrease of EHR was noted. Maximal EHR is reached when morphological development of the embryonic heart is completed. Furthermore, we examined 27 embryos with low EHR compared with greatest embryonic length. All of these ended in missed abortion within the embryonic period. According to our data the increase of EHR parallels the morphological development of the embryonic heart. Decline of EHR afterwards is interpreted as a functional adaption to muscular development of the heart.