I bought a Heart doppler!!

So today my new fetal Doppler arrived and I am so excited to hear my babies heart beat. Took some practice but I think I found it.... best to lie down flat and start low down by the public bone and work up as I found my babies lower than I expected. Be careful you don't get confused with the blood rushing through the placenta as well. I searched for both so I knew I had found the right thing.

I am wanting to sit and just listen to baby all day but I read somewhere it's not good to use it for longer than a few minutes each day. Is this true?