I lost a few kg in the first tri and I have just reached my starting weight again now at 18.5 weeks.
This happened to me last time and I was feeling pretty smug being half way with no net weight gain, but somehow I managed to put on 20kg in 20 weeks! I don't really want to repeat that (considering I kept the last 5kg of it) so I am being a bit more careful about what I eat this time (starting tomorrow....)