Totally not worth the investment. Sound quality is terrible, very fuzzy. Your better off investing in a Doppler. I bought one off eBay 42. It's simalar to the one your doctor uses and mine came with gel: oops! I thought you were talking about the thing you listen to the heart beat: I don't know about the other:
I have been told that they aren't recommended because your amniotic fluid amplifies the sound coming from them. Instead just listen to music as you normally would without headphones and as your baby's ears develop they will be able to hear it just fine.
Re: Belly Headphones