A fetal Doppler is a handheld ultrasound device that looks like a small radio with a transducer attached. Doctors use it to listen to a yet-to-be-born baby's heartbeat. For most women, their baby's heartbeat should be detectable via Doppler by 12 weeks of pregnancy -- the end of the first trimester. However, there's a lot of individual variation in when the heart will become detectable.