Big Mommies Make Big Babies: Greater Pregnancy Weight Gain = Higher Birth Weight


Here’s a big one for the pile on about gaining too much weight in pregnancy: A new study says moms who gain too much in pregnancy are much more likely to have big babies. And big babies, the author of this study suggests, may be at higher risk of future obesity.

Plus, big babies are harder to get out.

I know what you’re thinking, or at least what I was thinking. How do they know this isn’t just a normal genetic disposition towards bigness?

To isolate genetic factors from the direct result of pregnancy weight gain, the subjects were mothers who had more than one child, and gained different amounts of weight in each pregnancy. The study polled more than half a million women, comparing the birth weights of more than a million of their children.

The results were clear. The more a mother gained, the bigger her baby was likely to be. The results are averages, so there are obviously many, many exceptions on all ends. But the numbers were consistent enough for the study authors to claim a direct relationship between a moms’ weight and her baby’s.

They were even able to quantify this increase:

“every additional kg gained by the mother increases birthweight by about 7·35 g. Infants of mothers who gained 20—22 kg, and more than 24 kg, weighed roughly 100 g and 150 g more, respectively, than did infants of women who gained 8—10 kg.”

That last number would be the equivalent of 17-21 pounds. Which is quite a bit less weight gain than an average woman has been told to aim for in pregnancy. The last person I can remember talking about gaining 17 pounds in pregnancy was Ben Stiller’s wife, Christine Taylor. I also remember her saying her favorite “treat” during pregnancy was cottage cheese and fruit.  My mother only gained 17 pounds when she was pregnant with me—by smoking instead of eating.

The study measured for weight gain rather than BMI, which some critics find problematic. There’s also the question of starting weight, which doesn’t seem to have been factored in here. We already know there’s a connection between moms who are overweight before pregnancy and her kids’ weight. So if many of the moms were overweight to begin with,  this could have affected the results. It also might explain the 8-10 kg window of weight gain, as overweight women are advised to gain much less weight during pregnancy than average weight women.

I knew that really big babies were at risk of complications and sometimes potential future health problems, this is the first I’ve heard of such a low threshold. The cutoff for “big” here is 8.8 lbs, which while not tiny, doesn’t seem all that gigantic to me. Average U.S.  birth weight is about 7.5 lbs.

Interestingly, the average U.S. birth weight actually decreased in 2010, for the first year in almost five decades.

photo: Herkie/flickr