There are lots of reasons why a baby might be born a little smaller than average. In general, newborns' birth weights can vary by quite a bit, and depend on a lot of interdependent factors. Some of those factors are believed to be environmental; exposures that the fetus may have experienced in utero. But now researchers are attributing some of those exposures to the actual environment itself — possibly even climate change. Can global warming affect babies' birth weights? Scientists aren't ruling it out.
On average, newborns in the U.S. weigh somewhere between 5 and 9 pounds at birth, with 7 and a half pounds being the rough average, according to the American Pregnancy Association. Newborns actually tend to lose a little bit of weight in the first week or so of life, and how much their weight changes often has to do with whether they're breastfed or formula fed, according to the World Health Organization. That being said, birth weights that are outside the average range can be fine, and babies are often healthy on either end. But sometimes, babies that are born quite a bit smaller may also be underdeveloped, particularly in terms of their respiratory function. But even babies born prematurely can do just fine if they receive adequate treatment while they continue to mature outside the womb in the first few weeks or months of life.
Babies who are twins, triplets, or part of another multiple birth also tend to be smaller, as do babies born to mothers who may not have had sufficient prenatal nutrition or care, mothers who smoked, or mothers who had chronic health problems. But there are some cases when babies are born at a lower weight, and the reason why isn't immediately clear.
New research that has linked exposure to extreme temperatures to lower birth weight in newborns is pointing toward a rather surprising potential cause: global warming. A recent study by the National Institutes of Health found that when moms were exposed to extreme hot or cold temperatures during their pregnancies, their babies tended to smaller. The risk for a smaller baby was increased the more time the mother was exposed to such temperature extremes. The study indicated that "low birth weight" meant the babies were born at less than 5 and a half pounds, according to the NIH. Babies born early often fall into that weight range, but the low birthweight babies in the study were born near or at full-term. This led researchers to consider that pregnant women should be wary of how much time they're spending in extreme temperatures.
Of course, as climate change impacts temperatures around the world, causing some fairly drastic swings in some places, how reasonable is it to expect that women can stay in a more temperate climate for the duration of their pregnancies? 2016 was the hottest year on record, according to NASA and the National Oceanic and Atmospheric Administration — meaning that last year was the third year in a row that the hottest year on record was exceeded. Climate change has caused our global temperature to rise about 2 degrees Fahrenheit since the late 19th Century, according to NASA. When you look at the industrial boom, transportation advances, and the developed world's collective dependence on oil during that century or so, scientists say it's pretty obvious why the world is getting hotter, and they've been advocating for something to be done to curb the effects for years.
While people know that global warming has the potential for major, irreversible damage — much of which will lend itself to natural disasters, rising sea levels, and perhaps even the increased spread of disease — this new research indicates that it could also be changing humans at an evolutionary level. If pregnant women continue to endure the effect of global warming, causing them to have smaller-than-average babies, what are the long-term implications? And why do these temperatures have an effect on fetal development to begin with?
Scientists aren't entirely sure on either front — but they do think it's possible that blood flow to the uterus could be restricted by the narrowing of veins in response to extreme temperatures. This is a natural physiological response; the body uses vasodilation and vasoconstriction to help regulate the body temperature in relation to the temperature in our surrounding environment. A reduced blood supply, especially for prolonged periods of time, could certainly influence the fetus' growth. But if temperatures continue to change, will our bodies' responses have to adapt accordingly? And could those new norms, in turn, influence the size of our offspring?
There are still a lot of unanswered questions, and more research is needed. As many women know, pregnancy in and of itself can also cause bodily temperature swings thanks to hormone fluctuations. And while a good air conditioner might be helpful during the summer months to take the edge off for expectant moms, fixing global warming, unfortunately, won't be such an easy fix.