I am a very anxious person and I was advised not to get one because of false alarms. I did not listen and got one anyway, I have the small pager type one that goes on the nappy. When there is no movement detected after 20 seconds it vibrates and tickles her tummy, if still nothing it will alarm.
I ummmed and arrred for days and in the end went for it.
To be honest I have had plenty of false alarms but it doesnt make me worse, I hear it tickle her a few times, it may be that the sensor has come away from her body or she has held her breath but I lie there and listen to the tickle and then hear her make a noise or move and I just feel relief. It has even alarmed a fair few times and again I hear her making noises or moving so i I do leap up but just as not to let it wake her more than me being scared that its gone off.
A few times it has fallen off the nappy which is why its gone off. I am not sure about the under matress ones, just telling my experience on the one I have.
I asked my mum in the end and she said knowing you I know you will get one no matter what I say but I do think you should as I know how anxious you are. I just hope and pray it never goes off for real *touches wood*
I cant advise to go for it or not, all I can say is that it does put my mind at rest and I'd rather have false alarms than not. If that makes sense. Most of the professionals say not to bother, I guess its just personal choice and how worried you may get with false alarms. All I know is it puts my mind at rest and prevents me checking on her every time I can't hear her.
Do what you feel you need to do is the advice I give.