I also lost weight in the 1st Tri (4kgs) but put it back on in 2nd and have added another 5kgs. My friend pre-preg was 55kgs and ended up 80 by the end (put on heaps in 1st tri)....but lost 15kgs in first two weeks via breastfeeding....
I read that of all the weight we put on during pregnancy, we put on roughly 10% of it from 0-3 months, 25% 3-5 months, 45% 5-7 months and the last 20% in months 7-9...but who knows!