As a mom who had 2 losses after our first daughter, my husband & daughter bought me one at Babies R Us (Cant remember the name of it but it was great) when I became pregnanct with our 2nd daughter. It was about $50 and we used it for that pregnancy and for our 3rd daughter. I want to say it was a Graco brand. Either way I asked my OB/GYN and my Dad (a Pedi) and they said there was nothing wrong with using them. I liked having it especially when baby wasn't moving/ being quiet or when Big Sissy was "talking" to her new sister (she was 5/7 for the pregnancies).
For me it was a nice reassurance that everything was ok :)
Basically I am saying don't rent the medical grade, get your own - you can use it anytime then :)