Afternoon diddygirl. If it is any reassurance at all, I had exactly this at my last (32) week appointment. MW put the heart monitor on my tummy, reading came back 115, so she left it there for a few minutes (well, she chased the baby about a bit too) and it picked up to about 160. She said to me that it was probably due to the baby clutching at the umbilical cord, as you were advised, and not to worry about it. I am now obviously unnerved because I don't want to think that anything negative will happen to my baby, but in this I realise that I have to trust my MW, who has seen it all and much worse before.
If your MW was in any way concerned, she would have kept you in for a period of prolonged monitoring. I haven't had the same 40 minutes of monitoring, or even 25 as you had, and on the one hand I'd've liked to have been, but on the other hand it means I haven't heard the fluctuations in foetal heartrate that you did (I am a worrier, this may have worried me more).
I even toyed with the idea of getting one of those home foetal monitor devices, but the stress I would be under if I couldn't find a heartbeat at all has put me off. My baby is kicking as and when it feels the need, sometimes all day, sometimes hardly at all, and I just have to keep reassuring myself that it is doing what it will and I will have to wait for my 36 week appointment before hearing from it again.
At 31 weeks gestation, your baby's reduced movement may be symptomatic of a reduced space to swim about in rather than a heart abnormality (which hopefully would have been picked up on the 20 week scan). Mine is definitely more up for elbowing me these days rather than somersaulting (I will be 33 weeks tomorrow), and prefers gently stretching it's toes under my ribcage rather than sudden kicking. Sometimes, what with all the braxton hicks contractions I have been having, I am relieved to feel any movement at all, at other times I have to remind myself how much I love to feel it moving as it merrily jabs me in the same spot for the umpteenth time in an hour.
Sorry this is such a long response, and I hope that you feel better knowing you're not the only one. x