I am 5' 8" and started my first pregnancy at 70 kg and lost about 5 kg after the 1st trimester. I only gained a few, maybe 6 kg by the end of the 2nd, and then gained about 22 kg in the 3rd. I wasn't doing anything different and I was walking regularly and not "letting myself go." I don't trust the weight-gain rates they put in all the books. I think you can tell if you're being reasonably healthy. My second pregnancy, I gained a lot less and also started out a lot lighter but I still had the same weight gain pattern as in the first with it all being in the 3rd trimester. Everyone's different I guess.