I've posted on this before. I've been trying to not let my weight go up too much as DD1 was v.big - but not that surprising as I ate loads when pg with her. I had to diet hard to lose any of the baby weight. This time I've tried to be more disciplined, although with a couple of serious lapses this week. But I'm way off the chart for weight gain. 
The midwives don't seem to be all that worried, saying "It's all baby" but somehow that isn't particularly reassuring. Don't have gestational diabetes, at least not yet, but bump is pretty big - and not due for over 2 months!
Does anyone know if you're supposed to keep gaining weight at the same rate in the third trimester?