Ultrasound pulses are waveform packets with well defined frequency characteristics.
Measuring the frequency shift between pulses when they are launched and when their echoes are received provides a way to determine the velocity of the moving structure. Typically, frequency shifts are sampled about 2000 times per second during ultrasound imaging, leading to a very precise way of monitoring patterns of flow velocity. Doppler features tell us a lot about the pump function of the developing heart, including an indication of when a heart is beginning to fail mechanically. Using Doppler signs as early as a month after conception lets us identify with high reliability which pregnancies will continue and which ones will not.