I think this just happened to me this week too. It's my first baby, and I'm also 31 weeks. I can feel movement lower down, bump is definitely bigger, and the thumps suddenly feel like a person banging on my tummy to get out, not just the little fish that was there before. Weird! can't believe there are 2 more months to go, it seems like a long time to have such a big baby inside me, pressing constant buttons on my bladder!
It doesn't feel worrying, and I'm not seeing midwife again till 34 weeks, so I'm interested to know if this really is the baby engaging (?), or something else?
Duzida - it could be, but it's much more likely that your baby has developed enough muscle to stretch out (which happens about 30 weeks). They then kick the crap out of you constantly and it feels like you are running a washing machine spin cycle 20 minutes after you eat chocolate (try it, it's very funny).
You need to lie on your side to make it easier when the kicks get very hard!
LeninGrad repeat after me 'the muscles recover' My stomach is very flat and quite toned now (DS is 11 months and DD two and a half so it's not take me too long ).
Gotta admit I am not surprised the baby does so much wiggling and jiggling about inside, I mean it must be pretty cramped in there. Apart of me will fancy pulling the baby's limbs straight after it's born and giving it a jolly good stretch. Maybe use a rolling pin on it?
Did you know that's why the ancient Jews used to use swaddling cloths for their new borns? It was in order to straighten the baby's limbs.