Birth order linked to 150 health conditions in study of 10 million American siblings

Whether someone is the older or younger sibling may be linked to certain health outcomes later in life, according to a massive new study.

Analyzing medical data from 10 million U.S. siblings over two decades, researchers from the University of Chicago and Columbia University found links suggesting that birth order may be connected to specific long-term health risks.

Researchers examined 569 disease categories across families and among siblings, identifying 150 conditions that were significantly associated with birth order.

LONGER LIVES, WORSE HEALTH? SCIENTISTS UNCOVER TROUBLING TREND ACROSS GENERATIONS First-born children showed a higher likelihood of neurodevelopmental and psychiatric diagnoses, including autism , ADHD, Tourette's and OCD, as well as immune-allergic conditions like food allergies.

Second-born children were statistically more prone to migraines , shingles, substance use disorders and gastrointestinal issues like gastritis.

Discussing the findings on "Fox & Friends," Dr.

Marc Siegel, senior medical analyst for Fox News, noted the value of such a wide-scale study.

"I love this study because it opens a lot of questions more than it gives answers, but I think I have some answers," the doctor said.

CLICK HERE FOR MORE HEALTH STORIES Regarding the heightened prevalence of ADHD among first-borns, Siegel offered his own hypothesis about parental behavior.

"My theory, they get more stimuli.

The parents are like hovering over them, helicoptering them ... and they don't know where to look," he said.

TEST YOURSELF WITH OUR LATEST LIFESTYLE QUIZ Addressing the disparities in allergic conditions , Siegel pointed to early environmental exposure, stating, "I think it's because the firstborn's immune system hasn't seen the environment before.

And if it's not set properly, you get an allergy." Siegel also touched on biological and psychological factors when evaluating differences between siblings.

"I think the womb changes after you have your first child, and the second child gets a womb that's already been used.