8 1/2 weeks with this pregnancy, I used a doppler as my kids didn't move at all (snoozy bears in the womb), and like you, mine were IVF, so I never thought they would happen and found great comfort hearing a heart beat. I haven't used it very much with this pregnancy, however just this week used it after bubs didn't move for a day (or so I thought) and I was able to instantly hear bubs heartbeat. I have the same one as my OB and bought it off ebay.

My OB said it was fine as long as it wasn't that often, I think the consideration is how much fat you have on your tummy - if you are a larger person, then it will take longer to hear it (as the doppler has to get through a bit more before it can 'hear' the wee little hearbeat). I think generally they would say wait until 15/16 weeks so you aren't stressing if you can't find it, but as has been said, it's much lower than you expect it to be. If you do it too early, you will get confused between your own pulse and the bubs, hence it's worth waiting to make sure you aren't worrying about the 'heart beat' being too low, when infact it's your own pulse you are hearing! Some people find it more stressful to have one than not to have one, only you of course will know which side you sit on.