I seem to remember 5lbs to be the average/recommended gain in the first trimester. At almost 11 wks I've gained 3.5 -- considering how much I'm eating, I find that very surprising!
In the 6 months prior to conceiving I lost 35lbs that I'd been holding on to since my first baby's birth in 2006. It is such a relief to finally make progress on that front so it does feel a bit touchy to turn around and put weight back on, but I know it is for the best reason in the world. I finally lost the extra weight by following a Paleo/Primal eating plan, which is basically no grains, no sugars, limited fruit, unlimited meat & fat & veg, some dairy if tolerated. I lost quickly enough to stay motivated and, most importantly, never felt deprived or hungry. Once baby comes I will get back on the plan. For now I am eating more grains & a ton of dairy, but still focused on lots of fat (butter, coconut oil, tallow) for satiation and for growing my little beans healthy brain.
I also remember a significant of weight melting off very quickly with nursing.