When you become pregnant weight gain is normal. You will gain roughly around 10-11 kgs. The baby and placenta may contribute about 4-5 kgs. Deposition of fat in breasts, maternal tissue may cause about 4kg weight gain, while the remaining weight will be from the water bag and other fluids.
If you are deviating from the normal weight gain range, causes for swelling in your body such as high blood pressure should be suspected. If there is no swelling it can be due to a large baby, large amount of water in the water bag due to diabetes mellitus, twin pregnancies, and abnormalities to the baby(such as no head being formed, or no G.I. tract being formed) can be suspected.
It is important to eat a healthy diet and to watch eating too much sweet, even though you have the cravings. A healthy weight gain will benefit both mother and baby as during pregnancy nutrition is given to the baby through what the mother eats. However there is no necessity to eat for two or more! It is only during the third trimester that the baby gains most of the fetal growth.