31 weeks and baby weighs 1800g according to the grow chart. i have put on 3 stones on myself since start of preg. is that good or bad for me and baby?

Problem for you: as long as you are not diabetic and the babys growth is in normal range, your weight isnt bad for the baby. However, going back to a healthy weight after pregnancy is harder if you gain a lot. For a large frame normal starting weight, about 30, At over 40 pounds, it may take you longer. Just be sure you are being monitored for diabetes and thyroid condition.