How the Weather Could Affect Your Baby's Weight

There are plenty of factors that could affect your baby's birth weight—and according to new research, weather could be among them.

Pregnancy in Hot Weather Lolostock/Shutterstock
Unusually hot or cold weather during your second or third trimester may put your baby at risk for a lower birth weight...even if he or she isn't born prematurely, according to new research. 

Researchers at the National Institutes of Health looked at information from 220,000 babies born at 19 U.S. hospitals. They studied birth weight patterns between 2002 and 2008 and weighed it against weather data. The researchers defined unusually cold weather as temperatures below the 10th percentile of temperatures in any particular region, while unusually high temperatures were defined as anything above the 90th percentile. The researchers identified babies born after 37 weeks gestation and weighing in at less than 5.5 lbs as "term low birth weight" babies.

After calculating average temperatures for each trimester of pregnancy and the duration of the entire pregnancy, researchers came to a surprising finding: Women who were exposed to unusually cold weather during their second or third trimesters had an 18 to 21 percent greater chance of having term low birth weight babies, while mothers who were exposed to extreme cold throughout pregnancy had a 257 percent greater chance of giving birth to low birth weight babies.

On the other hand, moms who were exposed to particularly hot temperatures during their third trimesters had a 31 percent greater chance of having low birth weight babies, while those exposed to hot temperatures had a 249 percent increased risk.

This study, which was published in Environmental Research, is unexpected, to say the least, and researchers aren't quite sure why this link might exist. With that being said, we've seen several other studies that link environmental conditions to birth outcomes as well—birth season has been linked to allergy risk, learning disability rates and baby's lung capacity.

It's possible that researchers simply observed a correlation, which doesn't necessarily prove cause-and-effect. Still, it might be smart to avoid going outside more than you need to during unseasonably cold or hot days. After all, who wants to brave unpleasant temperatures while pregnant anyway?