The period of time shortly before and after babies are born is vital to their development and future health: research has shown that everything from a mother's stress levels and exposure to secondhand smoke, pesticides, and pet dander can affect a child's risk of illness down the road. But here's something that you might not expect to have such an effect: The month in which a child is born—or in some cases, conceived.
"The scientific literature goes back almost 100 years linking birth season to almost anything under the sun, from income to life expectancy to height," says Hannes Schwandt, PhD, an economist at Princeton University's Center for Health and Wellbeing. But Schwandt cautions that much of the research on this topic has not taken into account factors like the mothers' socioeconomic status or the length of pregnancy—both of which can affect when a woman gives birth, as well as the health of their baby.
Still, several recent studies (one by Schwandt himself) have found interesting correlations and potential implications for a baby's birthday. Here are a few surprising finds.
May conceptions and premature birth
Women who get pregnant in May (and are due in the winter) have a 10% higher risk of delivering their babies prematurely, according to a 2013 study by Schwandt and his co-author Janet Curie, PhD. The researchers speculate that high rates of influenza in January and February likely play a role, since it's known that catching the flu can trigger premature labor in women nearing full-term.
That doesn't necessarily make May a bad month for conception, though. "In some years, flu season peaks much earlier," says Schwandt. "In that case, women who conceived in earlier months would be at greater risk." Regardless, getting a flu shot is a pregnant woman's best bet at protecting herself and her baby from health risks, now or in the future.
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Autumn births and physical fitness
A boy born in November can run at least 10% faster, jump 12% higher, and is 15% more powerful than a child of the same age born in April, according to a 2014 study published in the International Journal of Sports Medicine. Overall, kids with fall birthdays appeared to be more naturally fit than those born during other times of the year.
One possible explanation? The authors suspect that mothers who are pregnant through the summer are exposed to more sunlight and therefore produce more vitamin D, an important nutrient for fetal development. The American Congress of Obstetricians and Gynecologists (ACOG) says that all women—pregnant or not—should get 600 international units of vitamin D per day. Most pregnant woman can get enough vitamin D via food and prenatal vitamins, according to ACOG, so the group doesn't recommend routine screening for vitamin D deficiency during pregnancy. But if you're concerned that you're getting enough, talk your doctor.
Springtime babies and multiple sclerosis
Low vitamin D levels during development may also put babies at increased risk of developing multiple sclerosis later in life. A 2013 Queen Mary University of London study found that vitamin D levels in babies born in May were about 20% lower than in those born in November; May babies also had twice as many autoreactive T-cells, potentially dangerous cells that could later turn against the body's own immune system.
Previous research has suggested that the risk for multiple sclerosis is highest for people born in May and lowest for those in November, and the study authors believe the "sunshine vitamin" could be involved. They point out that their findings only show an association between vitamin D levels and MS risk—not cause-and-effect—and that more research is needed on the benefits of vitamin D supplementation during pregnancy.
The bottom line
Don't panic if you conceived in May, or dub your kid a future Olympian just because you're due in November. Any effects that timing does have on a child's health won't be nearly as important as other factors you can control year-round—like eating well, avoiding cigarette smoke and alcohol, and getting regular exercise while you're expecting.
"Unless you have a personal preference, there is no 'best' or worst' month to get pregnant or have a baby," says Schwandt. "Taking care of yourself and getting proper prenatal care will make a much bigger difference than whether you give birth in January versus June."