From 6 weeks of pregnancy (sometimes a little earlier), an embryo about 3 mm long can be identified in the fertilized egg. From this same period, most ultrasound machines can detect the heartbeat of the embryo. If the heartbeat is not detected or is unclear when the embryo length (CTE) is 5 mm, a repeat ultrasound scan is indicated after a week. The absence of cardiac activity at this stage is not necessarily a sign of fetal distress or a non-developing pregnancy.
The numerical values of the heart rate in an embryo during an uncomplicated pregnancy gradually increase from 110-130 beats/min at 6-8 weeks of pregnancy to 180 beats/min at 9-10 weeks.