I use mine all the time, I started around 10 weeks..it was hard at first, but once I learned the noises it was easy. Sounds like a horse running, if it's above 130 it's the babies heart, if it's below that it's most likely yours. Your not supposed to use it for more then a few min about 10 or so...aboard try to keep it around 3 times a week. They say that's the best. Use lots of the lube that it comes with you will get better results. If you don't hear anything don't panic the baby moved like crazy. Good luck.
Has your doctor made you listen to your babys hb during an us? That is the same noice you should be searching for. They are safe.
I don't have one or intend to buy one.