I have heard many mom's to be complain that they can't pick up the heartbeat on those things!! My Dr couldn't even pick it up at 13 weeks with the professional one they use in the office! it says as early as 10 weeks but you know what...the average gestational age to be able to get a h/b on a doppler is between 12 and 14 depending on your baby's location, movement at the time and the mother's weight. The heavier you are the longer it will take to hear!
I wanted to rent one for my first pregnancy too but my dh said it would just worry me if I couldn't hear the h/b and I think he was right!
Wow dh was right!!! Just kidding.