From the Baby-Center - 31 weeks

Your baby's arms, legs and body continue to fill out -- and they are finally proportional in size to her head. She weighs about 1.5 kilograms and looks more like a newborn. She measures about 41 centimetres from crown to toe. You may notice your baby isn't moving around as much. Don't worry; she's just running out of space in your uterus. As long as you can feel her squirming, you'll know she's just fine. Believe it or not, she still has plenty of growing to do. You can expect her to gain at least 900 grams more before delivery. Your baby's organs are continuing to mature and she is passing water from her bladder: good practice for the urinating she'll do after she's born. Soon you can wonder what your baby is wishing for -- brain scans have shown that fetuses have periods of dream sleep around month eight.
You've probably gained around 1.5 kilograms this month. Gaining 450 grams a week is quite normal during the last trimester as your baby has a final growth spurt before birth. Has your sleep pattern changed? Find out why.