I would like to also chime in and refer you to the book mentioned before. I bought it when I had time to kill between an ultrasound and doctor's appointment and it offers so much information. I especially like reading other people's stories in it. The doctor has put a lot of research into what she is doing.
That being said, she recommends a LOT of weight gain in the beginning and her studies show that is is very benificial to babies. What I able to accomplish this? No. HAHA! As others said, I was sick sick sick. I am 19 weeks tomorrow and have gained 9 lbs, where I think the book says 20 lbs by this point
The last visit I had gained 4 lbs and 2 weeks and I think my doctor was pleased with that, but it kind of freaked me out in 2 weeks I almost doubled what I'd gained in the 17 previous weeks.
My doctor brought the idea of a c-section up to me. She just asked what my feelings were on it. I told her I'd heard a lot of horror stories as a nurse and I was really leaning towards the section. She said that her threshold for vaginal deliver of twins is very low. She said both would have to be head down, certain weight, etc. etc. I now have a placenta over part of the cervix that doesn't seem to want to move so that is grounds for a section anyway.
On the other hand, my SIL is having twins shortly and one is breech. Her doctor wants to go ahead and try it anyway and hope the baby flips after the other is born. He always wants to try the vaginal route because of the lower risk to mom afterwards.