Icing on the cake? Excess weight gain in pregnancy is mostly fat

We all know that women do and should gain weight in pregnancy. After all, they are growing a whole new human being! However, new research studies abound on how much weight gain is best for mother and baby. Now, researchers have extended this exploration into not just the amount of weight gained but the type of weight gained.

Before diving in to the study, let’s first divvy up the types of weight: lean mass and fat mass. Fat mass is just what you think it is: the mass of tissue from adipocytes or fat cells. Lean mass is everything else, including water and protein and the like. Your total body mass is your lean mass plus your fat mass. Usually, weight lifters and gym junkies talk about their body fat percentage when bragging.

When women gain weight in pregnancy, they gain both lean mass and fat mass. This is great because the additional weight is an energy source for the growing baby and mother. However, a glut of evidence has now shown that excess weight gain leads to more complications both during the pregnancy and afterwards, for both mother and baby. For instance, the risk of future diabetes and obesity are higher for both. In the June 2016 Gray Journal, a group of researchers from Case Western Reserve University in Cleveland, Ohio, published their findings on which type of weight women gain in pregnancy.

The authors collected data on weight gain for 49 overweight and obese women. The researchers used a fancy method to calculate the women’s lean and fat mass called air densitometry. Air densitometry involves stepping into a futuristic pod to measure how much air your body displaces and applying some high school physics equations like Boyle’s law.  The researchers measured lean mass and fat mass at the beginning and end of pregnancy.

At the beginning of pregnancy, the women in the study had an average BMI of 30 and they gained an average of twenty pounds. Overweight women (BMI 25-29.9) are recommended to gain fifteen to twenty pounds in pregnancy. Obese women (BMI≥30) are recommended to gain eleven to twenty pounds in pregnancy. When women gained more than the recommended amount of weight, this weight was more likely to be fat mass. Regardless of how much weight women gained (too much or just right), they gained the same amount of lean mass.

What does all this mean? A reasonable conclusion is that women gain a certain amount of lean mass during pregnancy, and then any excess weight gain after that is fat. As the authors point out, the fact that excess weight gain is fat might explain why it’s so hard to get rid of postpartum and why it’s associated with long-term obesity and heart disease. This study adds to the growing literature supporting the importance of healthy weight gain during pregnancy for long term health.