For me with my first child, he had an irregular heartbeat in the womb. (mamanatural.com)
This is a magical moment during pregnancy, and it also offers the opportunity of monitoring a baby's heartbeat for medical reasons. (checkpregnancy.com)
To really help keep the baby calm, you could also consider giving a stuffed toy or blanket that has a small box with heartbeat sounds embedded in it. (pregnancy-info.net)