I know every mommy and baby are different, plus U/S weights can be very off.
But at 34 and a half weeks my DD was weighing over 6lb 3oz, and I had gained somewhere around 20 or so pounds. But I gained very rapidly at the end because I had pre-e. But I do know the weight estimate was almost perfect, at 35w 6d she was born at 7lb 12.5oz and 21in long.
Healthy weight gain amounts very from person to person. It depends on your weight (really bmi) before pregnancy, your eating habits, etc.
People who are overweight prior to pregnancy often change their eating habits to more healthy ones because they become more aware what they are eating and are concerned with eating healthy to help their baby out. THus they gain weight more slowly....really even losing weight off their body but still providing adequate nutrition to their baby with their healthy eating habits.
Moms who are sick in the beginning of a pregnancy may lose weight making their overall gain look lower than it is. For example...you lose 10 lbs at the start of your pregnancy due to morning sickness and throwing up. Your doctor looks at your weight at 30 weeks and sees you gained 10 lbs over your starting weight..before you were sick and lost. Well really you gained your 10 lbs back PLUS 10 in addition so you have gained 20 lbs in effect.
A tall person can be perfectly fine gaining more weight than a short one...I am about 6 ft tall and 40 lbs on me barely shows up where as 40 lbs on a 5 foot person will be much more noticable.
Even a person's metabolism makes a difference. Sometimes very young moms will have a naturally faster metabolism than older moms. Therefore the younger will gain less weight.
So as long as you are honestly eating a healthy diet, taking in adequate nutrition and a reasonable amount of calories...you should be fine.