you should talk with your doctor for advise specific to you.

generally speaking however it really depends on how much you weighed before you fell pregnant in the first place.

My first pregnancy I lost 5kg in my first trimester (because of healthy eating) and my doctor was please suggesting that I shouldn't aim to loose any more but should certainly aim to main tain my weight during pregnancy (giving me overall weightloss). I was thirty kg overweight to begin with. My second pregnancy I started within my healthy weight range and was told to aim to gain no more then 15kg.

My SIL was encouraged the whole way through her last pregnancy to loose weight (she lost about 20kg while pregnant), but she was about 50kg overweight prior to falling pregnant.