These are all questions to which every medical professional will have a different answer, unfortunately. If you have been doing prenatal visits with an OB, it is not unlikely that you will be weighed every time you have an appointment, and that there is a heavy focus on your weight gain. While it is true that gaining weight during pregnancy is necessary, and that eating so much that you gain too much weight too quickly is dangerous, I think it is reasonable to focus on healthy eating, and general health, rather than weight. Some doctors say that every pregnant woman should gain at least 11 pounds. Depending on the weight of your baby, that is about right, because your baby and its placenta, together with the amniotic fluid in which he or she is swimming around, should weight around this amount all by themselves by the end of your pregnancy.
As a rule, women who are underweight to start out with would gain more weight, while obese women (who would still gain some weight during pregnancy) are going to gain a little less. Well, the best advice I have got for you is to watch your food, and eat as healthily as you possibly can, and to get rid of the scales. Don't think that it is important to gain lots of weight to sustain your baby nutrition definitely plays a key role, but that does not mean you have to gain lots of weight to be healthy. In fact, when I gave birth, I was back to my pre-pregnancy weight within a few days. You don't have to add fat to your body to be healthy.