The body mass index (BMI) calculator calculates your BMI and let's you know if your weight is normal or not. Calculating your BMI before and during pregnancy is especially important as extremes of BMI (too high or too low) may affect your fertility and increase pregnancy complications. The  body mass index, or BMI, is a term that is new to most people. When compared to your weight in pounds (or kilograms) the BMI gives you a much better idea about your actual weight.
The BMI is the measurement of choice for many physicians and researchers studying obesity. The BMI is calculated with a mathematical formula which takes into account not just a person's weight but also height. The BMI equals a person's weight in kilograms divided by height in meters squared. (BMI=kg/m2).

In May of 2009, the Institute of Medicine (IOM) issued guidelines on weight gain during pregnancy. Depending on how much a woman weighed before getting pregnant, the IOM recommended weight gain for each category of prepregnancy BMI is as follows:

• Underweight (< 18.5 kg/m2); total weight gain range: 28 to 40 pounds
• Normal weight (18.5 - 24.9 kg/m2); total weight gain range: 25 to 35 pounds
• Overweight (25.0 - 29.9 kg/m2); total weight gain range: 15 to 25 pounds
• Obese (≥ 30.0 kg/m2); total weight gain range: 11 to 20 pounds