Doppler Effect - Electronic Stehtescope

Hey,
Anyone out there familiar with the doppler effect?
My idea was to build something like an electronic stethescope, but lean more towards the thing they use when listening to a fetal heartbeat, ya know? I am actually a father-to-be, and out of anticipation, my wife and I bought an at-home heartbeat monitor (which sucks!) and the thing barely even registers my own heartbeat.
Anyways, what I was thinking was improving the at-home heartbeat monitor. I thought about just buying the type that doctors use (the at-home versions) and those things are way too expensive for my minimum wage. So I decided why not just make an improvement? I don't wanna experiment on my unborn kid or anything, but if I can get my own heartbeat to sound like what I hear at those monthly doctor visits, that would be good enough for me.
Just as well, it would definitely further my knowledge of circuitry and it would be fun to delve into a new subject for me.
So, any ideas?

I have just been playing with a piezo (the flat type you get in a music xmas card) as a sensor to detect a squirt of water from a water pistol, and found them to be super sensitive.

I needed to feed it through an opamp to increase the gain, but think it might work well enough for your application.

Also very cheap.

Pete

It might be difficult or expensive for you to find powerful ultrasonic transducers to make an ultrasound machine to hear (or even to view) your new baby.

I recently had my heart and arteries tested with a modern electronic device that made all kinds of cool noises. I could never make one like that.
A while ago I heard my heartbeat loud and clear with an electret mic from a toy fastened to the middle of a plastic jar lid and a cheap LM386 amplifier driving cover-my-ears headphones.

Awesome. The squirt gun project actually sounds kinda fun!
Actually, just recently I was working on a guitar amp (with a 386) and used a piezo on the input. I was suprises at how sensitive it really was, and of course it sparked a whole new group of ideas. Although I would love to hear my babys heartbeat whenever I wanted, he is only a few weeks away and I would not want to experiment with ultrasonic waves on him anyways. But these are some great ideas so far! Thanks!