Baby Hearbeat App?

Have any of you used the baby heartbeat apps your phone offers? I'm 17+4 and starting to get nervous. I still don't look pregnant and have felt nothing that I'm positive is movement. This is my first pregnancy so I know things happen later the first go around. I feel great and I'm sure all is well, but I'm a worrier :) My next appointment isn't for another 3 weeks and I'm just needing reassurance! Thanks in advance for your feedback!