Regg, congrats haha...
As for weight gain, it's a guidance la. As long as u remain healthy and not develop symptons that might lead to complications. I think the belief is more that the slightly plumper gals already have more fats in their body pre-pregnancy which can be used during the pregnancy as opposed to the thinner gals that needed to put these fats on. I have a friend who lost so much weight during her 1st trimester, that she actually looked terrific during pregnancy (ie she was plump to begin with). So i think with rdgs to weight issue, no need be so stressed.
Like my gynae in Singapore who is super weight conscious kind, tell me I can only put on 10-12kg (BTW I am quite slim la). I got a hard time when I went back to Singapore to deliver my #1 and put on 1 kg in 2 weeks. He almost killed me and questioned me what did I eat. As long as u hv a supportive gynae and no complications, i think it's fine. Of course, dun put on too much, else hard to lose.