4
Aug 2026
Study maps how birth order tracks disease risk across 10 million siblings
Researchers analyzing claims from more than 10 million siblings linked first-born status to higher odds of several neurodevelopmental and allergic diagnoses, while second-born status tracked with substance abuse and gastrointestinal conditions. The observational study identified 150 significant birth-order associations across the phenome, but claims data and residual confounding prevent causal conclusions.