BMI and Pregnancy Weight Gain

person standing on white digital bathroom scale

How to use an easy BMI formula to figure out your recommended weight gain.

Even though it might seem that all of the pregnancy pounds go straight to your hips, thighs and buttocks, there is a purpose for the extra weight gain. Your body needs to actually grow more tissue.

Case in point – the placenta! Here is an organ that isn’t even there until the baby is conceived and implants itself into your uterus at around 6 weeks’ gestation. The placenta’s weight can reach 1 pound and even more for a larger baby.

The mother’s body also needs to increase blood volume by 50% during pregnancy which often adds at least 2 pounds to her overall weight gain. Here is how typical pregnancy pounds would be distributed:

  • Baby – 7 ½ pounds
  • Placenta – 1 pound
  • Uterus – 2 pounds
  • Amniotic Fluid – 2 pounds
  • Breasts – 1 pound
  • Blood Volume – 2 ½ pounds
  • Fat – 5 pounds
  • Tissue Fluid – 6 pounds

Many women will lose much of their pregnancy pounds in the immediate postpartum period and then gradually lose the rest over several months. It can be easier to think that it took nine months to gain the weight and that you should give yourself nine months to lose those pregnancy pounds. Bear in mind that if you plan to breastfeed, it is a great way to lose some of the additional fat stores.

Don’t forget that calories are important too so be sure you are getting enough as well as eating the right diet during pregnancy

Exactly how much weight gain is recommended for the average women to have a healthy pregnancy and baby? Does it make a difference how tall you are or whether you are over or underweight to start with?

Most experts agree that a normal range for recommended pregnancy weight gain is between 25 and 35 pounds. To be more exact, however, you can calculate your own recommended weight gain by using this formula for body mass index (BMI):

[Weight (pounds) / height (inches) / height (in)] x 703

Here is an example:

  • A woman weighs 150 pounds before pregnancy and she is 5’8 inches.
  • Divide 150 by 68 inches = 2.205.
  • Divide 2.205 by 68 which equals .0324.
  • Then multiply .0324 by 703 – 22.80.

This woman’s BMI would be 22.80.

Here are the following categories for BMI to determine if you are normal, over or underweight:

  • Underweight – less than 19.8
  • Normal weight – 19.8 to 26.0
  • Overweight – 26.0 to 29.0
  • Obese – over 29.0

(Institutes of Medicine,)

If you have a normal BMI, during pregnancy your recommended pregnancy weight gain is 25 – 32 pounds. If your BMI is in the underweight category, you should gain 27.5 to 40 pounds. If you are overweight, your pregnancy weight gain should be 15 – 25 pounds. Women who are in the obese category should gain approximately 15 pounds. Women carrying twins should plan to gain about 35 – 44 pounds.

So in a nutshell, be patient with your own pregnancy weight gain. Remember that those pounds serve a very important purpose by growing your baby. It is more important to eat the right foods that are packed with nutrients and vitamins than it is to gain the recommended weight.

How much weight have you gained? Share your experience.

For more information on eating healthy every day, read Organic Foods and Pregnancy and the entire Nutrition series.