Congratulations on your pregnancy. I didn't notice the waistline disappearing until about 4 1/2 months at which point I began to look as if I had put on a few pounds. I didn't look pregnant yet, just less slim than usual. This is the point at which I went out and bought a few maternity items. But given that this past summer was touting empire waist fashions for juniors, I was able to buy a lot of sale items out of the Juniors section of the store.
My biggest weight surge so far occured between 7 1/2 - 8 months--5 pounds in 2 weeks! There is no way I can wear those Junior shirts now as they are too short in the belly. I was able to wear them up until about 7 1/2 months.
I didn't gain anything until the early second trimester, but every woman is different. Some spread it out more evenly over 9 months. I'm one that moves in surges, and most of my gain has been in the third trimester. My first surge was around 6 months; then again at 7 1/2 months. I'm down to the wire now (Nov. 18) and gaining rapidly. So most of my weight gain has been at the end of pregancy. At 36 weeks, I'm up 27 pounds from my pre-pregnancy weight. But again, every woman is different depending on bodily frame, genetics, lifestyle etc. I have to say that I have certainly enjoyed this plump, round season of my life. And I have given myself full permission to love the pregnant, round, and full earth goddess that I have become. I say, embrace it, but maintain your good habits at the same time so that the amount you gain will be natural to your body.