In general, the Doppler has high power than ultrasound. So, avoid using them frequently during the first trimester of the pregnancy. In obstetrician’s clinic, they quite often use an ultrasound to check the image rather than sound. The sonogram uses the ultrasound to render images while this Doppler used to measure the movement of blood in the arteries (or heart-beat). So, one can see the baby’s heart beating in this ultrasound. Some models of ultrasound machines allow to hear the heartbeat even before using this handheld Doppler. Dopplers won’t detect the baby’s heartbeat reliably until 10 – 12 weeks but whereas the ultrasound gives better results and show your baby’s heart beating in an image.