Being born in certain months could make you more or less likely to experience certain illnesses.
Crazy as that might sound, there are reasonable explanations for these differences. Kids born in some seasons are exposed to different risk factors than children born in other seasons. There might be an abundance of dust mites and other allergens in certain months. People are more likely to be experience vitamin D deficiency in the winter, and if this is the case at the beginning of life, it could persist throughout life.
Of course, some (or even all) of the associations could also be statistical noise — the kind of coincidental correlations that can pop up when you're looking at so many different variables.
For a 2015 study, researchers at Columbia analyzed 1.75 million electronic medical records for people treated at Columbia University Medical Center between 1985 and 2013, and found 55 diseases with seasonal links (out of 1,688 diseases they looked at), including asthma, ADHD, reproductive issues, cardiovascular illnesses, and neurological problems. (This means the vast majority of diseases do not have a seasonal link.)
Being born in some months had a stronger overall effect than others: Being born in September, October, or November was associated with a higher disease risk, while people born in February, March, April, May and July were less likely to develop illnesses. Being born in May, June, August, January, and December didn't provide an "overall advantage or disadvantage," according to the study.
In the graphic below, you can see the number of diseases that people born in certain months are more or less likely to experience. So, for example, people born in October were more likely to experience several respiratory illnesses, but less likely to develop several cardiovascular diseases.
Keep in mind that these are probabilities. A November birthday doesn't mean you'll definitely develop a reproductive illness — not even close.
It should be noted that just because there are certain diseases people are more likely to experience in their lives, that doesn't mean there's a big effect based on birth month.
"The risk is not that great that you should worry about when your baby is going to be born or when you might have been born," says Nicholas Tatonetti, the principal investigator who explains the work in a YouTube video. "There's lots of other variables that account for disease incidence."
Diet, exercise, and stress level are all more important factors — by a long shot.
Also, this study is based on patients treated in a New York City medical center, which means that these particular charts and associations might be particular to New York residents.
Because researchers think that environmental factors like sunlight (because of vitamin D) are responsible for most of the variation in risk, these risk profiles would look different in places with different environmental conditions. Conditions related to vitamin D, for example, might have a mostly reversed profile below the Equator.
Even if these specific associations have a regional focus, the researchers say this work should help them identify environmental factors that increase or decrease disease risk everywhere.