Weight gain during pregnancy starts around the third month due to an increase in the size of various organs, like the uterus and the breasts. The total weight gain differs from woman to woman, but should not be more than about 10-12 kgs.
The first thing to do is to see that you are eating the foods which are part of a well-balanced diet. The "eating for two" concept is outdated. What is important is that you get adequate nourishment in the form of vitamins, proteins and minerals for your own and your baby's well being. Your doctor will prescribe vitamins, calcium and iron supplements to check deficiencies and the baby will help himself according to his requirements.