What can a body mass index calculator tell you?

The Body Mass Index (BMI) calculator calculates your BMI and lets you know if your weight is optimal. The amount of weight a woman needs to gain during pregnancy is based on her prepregnancy body mass index (BMI), which compares weight to height. In general, if your BMI (Body Mass Index) is low, you should gain more weight than if it's high.

BMI (Body Mass Index) Calculator

Why is body mass index (BMI) useful?

When compared to your weight in pounds (or kilograms), the BMI gives you a much better idea about your actual weight. It 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).

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.

How much weight should you gain during pregnancy?

The Institute of Medicine (IOM) recommended weight gain is as follows:

  • If you were underweight before pregnancy (for the IOM, that is a BMI of less than 18.5): between about 28 to 40 pounds or 12.5 and 18 kgs extra weight during pregnancy
  • If you were normal weight before pregnancy (for the IOM, that is a BMI between 18.5 and 24.9): between about 25 to 35 pounds or 11.5 and 16 kg extra weight during pregnancy
  • If you were overweight before pregnancy (for the IOM, that is a BMI between 25 and 29.9): between about 15 to 25 pounds  or 7 and 11.5 kg extra weight during pregnancy
  • If you were obese before pregnancy (for the IOM, that is a BMI of over 29.9) about 11 to 20 pounds  or between 5 and 9 kg extra weight during pregnancy
  • If you are very young, then more weight gain is probably needed as teenagers may still be growing themselves.

Research studies have proven obesity is a major concern during pregnancy. Pregnancy complications associated with pre-pregnancy obesity may include increased risk of C-section and excessive fetal weight gain. Much of the science on the matter is clear – women who are overweight or obese when they become pregnant are in the highest risk category, but what about women who start out the pregnancy in the normal BMI range, but end the pregnancy in the obese range?

Can a woman move from normal weight to obese in a healthy pregnancy?

If you look at basic numbers, a woman who is 64 inches tall weighing 140 pounds is considered to be a normal weight with a healthy BMI. Acceptable weight gain for this patient would be up to 35 pounds. If she gained all 35 pounds she would move from a normal to obese, in terms of BMI. She did nothing wrong, ate all healthy foods and gained the right amount of weight, but the numbers say she is now obese.

Does the obese BMI during pregnancy increase health risks?

For most women, the majority of weight gain occurs in the third trimester when the baby is growing by leaps and bounds. There is little formal research or even mention of pregnancy BMI, other than information for women who are overweight or obese (and underweight) prior to pregnancy. If you are of normal weight when you conceive and you eat healthy, exercise and gain the recommended 25 to 35 pounds, you are having a healthy pregnancy. The BMI change will occur for every pregnant woman. Even women who gain the lower end of the spectrum will have a BMI that measures in the overweight range at birth.

How to calculate your BMI during pregnancy

If you want to know how your BMI is holding during pregnancy, subtract the weight of pregnancy from your body weight before calculating BMI. During the first trimester, you should be gaining about one pound per month. That increases to 0.5 pounds per week in the second trimester (about two pounds per month) and one pound per week (about four pounds per month) by the third trimester. You can use these numbers to calculate your BMI. 

The 5’4” woman who weighed 140 pounds at pregnancy was on the edge of being in the overweight category. By the end of the first trimester, she should have weighed about 144 pounds. She’d reach 150 pounds by the end of the second trimester and about 160 to 165 pounds by the end of her pregnancy. Taking those healthy weight gain numbers into account, if she gained 35 pounds during pregnancy she should lose about 20 to 25 pounds a short time after giving birth, leaving her postpartum weight at about 150 pounds or about 10 pounds over her healthy body weight.