What's the Healthiest Month to Be Born In?

What's your sign?

Mary Regina Boland is a Halloween baby -- a scary birthday not only for the superstitious, but in some ways also for the health-conscious. In recent research, Boland, a Ph.D. fellow at Columbia University Medical Center, and colleagues found that babies born in October and November had among the highest risk of developing diseases, while those born in some spring and summer months were generally better protected. That's not pseudoscience, since seasonal factors like vitamin D exposure during pregnancy can affect health outcomes. "It's not horoscopes or the alignment of planets," says John Perry, a senior investigator scientist at the University of Cambridge. Your birth month really can reveal things about your health.

Allergies and asthma

In Denmark, people born in May and August are more likely than those in other months to have asthma. In New York City, where Boland's team conducted research using 1.7 million patient records, those peaks are shifted two months, with July and October babies at high risk for the condition. That makes sense, since peak sunshine -- which can trigger dust mite allergies and affect asthma -- is about that much later on the East Coast of the U.S., Boland's research confirmed. "The high-heat, high-humidity mechanism causes a lot of dust-mite exposure, and being born in the high dust mite environment can increase your risk for developing allergies later in life," she explains.

Reproductive problems

Boland often hears women discussing their desire for a spring or summer baby. They may be on to something. "May, June and July ... seemed to be a better time to give birth," Boland says, when it comes to babies' overall disease risk. Women's own birth month seems to be linked to their reproductive health, too, with those born in October and November at highest risk for some childbirth complications like false labor, she says. Still, rather than fixate on your or your baby's expected birth month, focus on well-established ways to support a healthy pregnancy, such as taking prenatal vitamins, staying physically active and eating a healthy diet, Boland suggests.

Heart conditions

Boland and fellow October babies can take heart at this finding: They are the most protected against cardiovascular diseases, followed by their September and November neighbors. People with March and April birthdays, meanwhile, are at highest risk for heart trouble, Boland's research found. One theory: "Some people think that being exposed to the flu virus when you're pregnant ... can damage [the baby's] heart in a small way," Boland says, although more research is needed to figure out whether that's true. Meantime, women who are pregnant during flu season should talk to their doctors about getting the vaccine, experts say.

Multiple sclerosis

While spring baby girls catch a break in the reproductive health department, men and women blowing out candles in April and May are less lucky when it comes to autoimmune diseases, namely multiple sclerosis, many studies of people in the Northern Hemisphere have found. That seems to be because women giving birth in spring tend to miss out on sunlight -- aka vitamin D -- during their third trimester, which influences their future child's immune system and MS risk, research by Sreeram Ramagopalan, an epidemiologist at the health care company Evidera, has found. The findings, he says, highlight "just how important vitamin D is for the immune system."

Attention deficit hyperactivity disorder

As a July baby with a doctorate, Perry admits he's "bucking the trend." His research has found that Brits with summer birthdays, namely August, are less likely to go to college than those born in the fall, particularly September. But the reason seems to be more practical than biological, since school-age cutoffs make summer babies in the U.K. younger than their classmates. Boland's team noticed a similar trend in the U.S. when it came to developmental and neurological conditions like attention deficit hyperactivity disorder, which was most prevalent among boys born late in the year, when New York City, for example, draws the enrollment line.

Nearsightedness

If you've ever been called "four eyes," chances are you celebrate a summer birthday, research by Dr. Yossi Mandel, head of the Ophthalmic Science and Engineering Lab at Bar-Ilan University in Israel suggests. "Babies born at summertime, where the daylight time is longest, have about 24 percent higher risk to be severely myopic," or nearsighted, presumably since they see more of the sun early in life, he says. While there's no consensus on how best to prevent nearsightedness -- a condition that's increasingly prevalent worldwide -- some recent studies suggest that spending more time in the sun as a kid can help, Mandel says.

Body size

Perry, an underweight baby who grew into an average-height man, also bucks trends when it comes to body size: His research has found that summer babies tend to be born heavier and grow taller, and girls go through puberty later -- all factors linked with better overall adult health. In this case, too, vitamin D exposure during pregnancy seems to be at play. "It's hard to think the amount the mother goes out in the sun has an impact on a baby's health in two decades' time," Perry says, but it does. Still, factors like genetics matter more. "Where you're born in the year," he adds, "isn't going to explain why you're 5-foot-2, not 6-foot-8."