My Wife Is 7 Months Along In Her Pregnancy And

my wife is 7 months along in her pregnancy and a strange thing happens while she sleeps. she breathes in and out, and her chest heaves as usual but then her hips and belly heave. so her hips are heaving between breaths ??? it just looks really strange.. any ideas? this is our first child so i don't know if thats normal or not. Thank you