I wouldn't worry too much - is your Ob concerned with weight gain? With both my other ones I put on between 25-28 kg over the whole pregnancies. Apparently 'good' or 'recommended' weight gain is 10-15 kg, but screw that LOL I was active during both pregnancies, walked a lot, ate well, mostly (apart from giving into chips-cravings at the beginning when little else tasted good) and had two drug free, (relatively) quick labours with healthy babies. Plus I lost it very quickly after the birth, without having to do much at all, just breastfeeding and eating normally. The biggest concern they have with weight gain is if it's really quick gain (like you gain a few kilos in a week) and if you're getting pitting oedema as that can indicate other health problems. A good friend of mine gained 3 kg with her first, and actually LOST weight during her second, but she was a bigger girl to start with anyway - my starting weights were 59 and 62 with the first two and I've heard if you're smaller you often gain more than if you're bigger.

Hope this helps and I haven't made you worry even more! Don't forget there's lots of blood and fluid and stuff as well as the weight of the actual baby which contributes to your weight too. Plus you'll be gaining weight for breastfeeding stores (in my case, thunder thighs!!)