If you're overweight/obese, most doctors wouldn't care if you didn't gain any weight as long as the baby is measuring fine. I really wouldn't go crazy trying to gain weight because the baby will get everything it needs from you. Just make sure you're taking all your vites and eating a balanced variety of foods to get good nutrients. With my first pregnancy I didn't gain any weight until 26 weeks, but gained about a pound a week after that for a total of about 20lbs by the time I delivered at 41 weeks.
HW/SW/Original Goal/CW/New Goal Post WLS baby, born 11/10/11 and 3/20/14