One of the misconceptions most pregnant women hear is that they should eat for two. Wrong. If you do that, there's only hell to pay when the baby is out and you're still left with all the post-baby flab. Generally you only need to eat 100 to 300 calories more than you used to prior to pregnancy.

If you started out on your pregnancy being underweight, then you indeed have more room for weight gain rather than someone who's already overweight right from the beginning of pregnancy.

How much weight should you gain during pregnancy? There's no one-size-fits-all answer. From WebMD:

A woman of average weight before pregnancy should gain 25 to 35 pounds during pregnancy. Underweight women should gain 28-40 pounds during pregnancy. Overweight women may need to gain only 15-25 pounds during pregnancy. You may need to gain more or less weight, depending on what your Health care provider recommends.

To break this down, in general, you should gain about 2 to 4 pounds (that's about 1-2 kg) during the first trimester and a pound per week until birth. Having said that, pregnancy is not the right time to scrimp on food as you need to make sure that the baby you're carrying is getting all the nutrients s/he needs, plus a little extra for you. The key is eating healthy, balanced meals, with portions just a little bit more than what you used to have.