Your weight gain during pregnancy depends on many factors including your weight before pregnancy. Whether you were underweight, normal or overweight for your height before pregnancy will impact the weight that you gain during pregnancy.
If you are underweight you are expected to put on more weight during pregnancy.
If you were overweight before pregnancy then chances are that you will add comparatively lower weight during pregnancy. It is, however, strict no for any diet program during pregnancy.Most of the weight gain takes place during 2nd and 3rd trimester.
Weight of the baby is expected to be the same irrespective of your weight before pregnancy. The difference in weight gain for underweight, normal and overweight mothers usually comes from difference in gain of other elements such as fat, body fluid, maternal bllod, etc.