US study finds birth order shapes health risks for siblings

Technology Oct 12, 2026

Ever wondered if being the oldest or youngest in your family could affect your health?

A massive study of approximately 5.1 million American two-child families just found that firstborns are more likely to face neurodevelopmental issues and allergies, while second-borns have a higher risk for substance use problems and gut-related diseases.

Study: shorter sibling gaps lower risks

Researchers looked at sibling pairs and found that even with similar upbringings, birth order made a difference in health risks.

Shorter gaps between siblings seemed to lower some risks for both children.

While birth order isn't the only factor, these findings could help doctors create more personalized checkups and prevention plans based on where you fall in your family lineup.