I'm still pretty early in my pregnancy, so I haven't learned much about your 1st & 3rd questions yet. However, I was reading some nutrition guidelines recently and the suggestion was that someone who is pregnant with a singleton should gain 4lbs during the first trimester and then 1 lb per week throughout the 2nd & 3rd trimesters.
boy/girl twins arrived 3/22/07 via c-section
I started going every two weeks at 18 weeks, but due to a large fibroid. They wanted to measure cervical length each time. I think that the third trimester (27 weeks) is when you start going every 2 weeks, then at 34 weeks you go every week...