A fetus begins to develop a heartbeat around 5 to 6 weeks of pregnancy. At about 22 days after conception (around 3 weeks embryonic age), the primitive heart tube starts beating with rapid and irregular contractions. This early heartbeat can often be detected by vaginal ultrasound around weeks 5 to 6 of pregnancy, although it might not be audible with typical Doppler devices until later. By week 6, the embryo's heart rate is approximately 110 beats per minute and continues to develop, forming chambers and valves over the following weeks, with a fully developed heart by about week 10. The heartbeat detected early on is due to cardiac tissue pulsating, which eventually becomes the functioning heart of the fetus.
In some cases, the heartbeat might only be clearly heard around 8 to 10 weeks due to variations in gestational age calculation or development timing. The sound often heard at 6 weeks on ultrasonic devices is an electrical pulse translated into sound by the machine, as the heart valves and chambers are not yet fully formed at this stage.
