Here are the current US weight-gain guidelines (from Iffath Hoskins, MD, Chair of Ob-Gynat Lutheran Medical in NYC):
overweight women should gain 15 to 25 pounds
normal weight women should gain 25 to 35 pounds
underweight women should gain 28 to 40 pounds
(Men shouldn't gain anything, but it happens....)
There is currently a debate about the gains for overweight women because of the obesity situation in the US
And I thought this was interesting:
the fetus will weigh 7 to 7.5 pounds (okay, our baby was over 8)
the placenta will weigh 3.5 to 5 pounds
amniotic fluid will weigh 2 or 3 pounds
retained water will weigh 5 to 7 pounds
when the baby is born, the mom is usually left with about 1/4 of the weight gain