Fetal heart monitors

Lynzi • Mommy to Garrett Mitchell 💓 jan 1. 2016
Okay mommas, if you use fetal heart monitors when did they start to work? I just purchased one hoping we would be able to hear something but unfortunately not. I know it says in the packaging it works best past 20 some weeks and I'm only 17+4 but I thought I would be able to detect something. 
So I'm wondering when everyone else was able to hear a beat? Thank you!