A healthy amount of weight to gain for most women by the end of pregnancy is between 25-35 pounds. This number can vary due to many different factors. For example, if you begin your pregnancy overweight, you may only need to gain between 15-25 pounds. Likewise, a woman who is underweight at the beginning of her pregnancy may need to gain more.
Most of the weight you gain will be during the third trimester when the baby is growing rapidly. In addition to your growing baby, fat stores, excess water, increased blood supply and many other factors will contribute to weight gain throughout the entire length of your pregnancy.
No matter where you started, pregnancy is not the time to attempt weight loss.