It is important to gain the right amount of weight during your pregnancy. Both too much and too little are associated with long term complications for children and their mothers. The amount of weight you should gain, during pregnancy, is based on your weight prior to becoming pregnant. The body mass index (BMI) measures your body fat based on your height and weight. A normal and healthy weight is a BMI of 18.5-24.9. A BMI of 25-29.9 is considered overweight. Women who have a BMI, before pregnancy, of 30 or higher are considered obese.