Cardiac activity in a developing fetus usually starts at the end of the 5th week. So by 6 weeks you should be able to see a heartbeat via ultrasound and by 10-12 weeks you should be able to hear the heartbeat via doppler device,The embryo's heart begins to beat around 6 weeks of pregnancy – sometimes earlier, sometimes later. A transvaginal ultrasound (an internal ultrasound) can detect a heartbeat around 6 weeks of pregnancy. However, it isn't uncommon to be unable to detect a heartbeat via ultrasound until closer to 7 or 8 weeks.