Fetal heart monitors

Lynzi β€’ Mommy to Garrett Mitchell πŸ’“ jan 1. 2016 TTC, 1 ectopic pregnancy, 1 miscarriage, STILL trying πŸ’• πŸ‘ΌπŸ»πŸ‘ΌπŸ»
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.Β 
So I'm wondering when everyone else was able to hear a beat? Thank you!Β