Fetal Doppler at home?

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Hi ladies!!
 
I was wondering how far along you have to be to use a fetal Doppler at home, 
do they really work, 
and are there any recommendations on which one to get?
I had a miscarriage earlier this year and am 7 weeks with my little rainbow. And as you can imagine I am a nervous mama! I just want to know that little babe it doing ok!
Thank you in advance for your recommendations!! 💕💕