The amount of weight gain per pregnancy depends upon the pre-pregnancy weight of the mother. So it’s doesn’t mean that all the ladies have to be in the same amount of weight.
Any ladies obese, should not be gaining more than on average should be gaining around eight kgs. Who is overweight, should be around 10, a normal girl should gain 12 and underweight should be gaining around 14 to 16 plus-minus a kg or so.
Too much weight gain doesn’t indicate you’re going to have a very healthy baby rather than an obese lady who has gained too much weight, there will be increased chances of blood pressure and gestation diabetes.