During what trimester does a baby's heart start to beat?

early 1st trimester: A good technician with a good ultrasound can detect a beating heart at about 4-5 wks after conception, but i believe it starts beating much earlier than that. It is detectable at this stage, but not likely audible even with the sound amplification ability of the available ultrasound. It is currently a detection limit by the available technology we have. Have fun and good luck.