What Leah has mentioned above is pretty much what my baby book says too ~ but it is a guide only.

I started all my pg's pretty overweight & in my first pg put on 7kg to 32wks when my son was born (he weighed 1.738kgs), my second pg ended in a late loss at 18w5d and I had put on 4.5kgs to that stage (bubs weighed 250gms) & so far in this pg I haven't put on any weight.

The goal for me while pg is to try & maintain my weight & not put on too much because I am overweight before falling pg & once pg my BP tends to play up a bit.