Hmm, I'm not entirely sure which heartbeat apps you're referring to exactly, may malalim na kasi na available na ngayon, from fetal doppler trackers to fitness ones! If you're talking about fetal heartbeat monitoring apps during pregnancy, most of them work by connecting to a portable doppler device na sinusuportahan nila, pero honestly some of my mum friends said na hindi pa rin gaano reliable compared sa actual ultrasound sa OB. Best thing to do siguro ay i-check ang specific app instructions niya or ask your doctor kung which ones are actually worth using, kasi may iba na basta gimmick lang talaga. What kind of heartbeat app are you looking at po?