Does birth order define your health? Study of 50,00,000 families reveals disease link

A study of more than 50 lakh American two-child families found birth order was linked to later disease risk patterns. The findings suggest early-life experiences may shape health differently for firstborn and second-born siblings.

In Short

• Study examined insurance records from over 50 lakh two-child US families
• Researchers found birth-order links across 150 of 418 medical conditions
• Firstborns showed higher diagnoses of ADHD, allergies, asthma, anxiety and autism

Siblings may share the same parents, home and childhood, yet grow up with strikingly different health risks.

A study analysing medical insurance records from more than 50 lakh American two-child families has found that birth order is associated with differences in the likelihood of developing several diseases later in life.

Published in Nature Health, the study led by Andrey Rzhetsky, Edna K. Papazian Professor of Medicine at the University of Chicago, examined 418 medical conditions.

Researchers found statistically significant associations between birth order and 150 conditions, suggesting that early-life experiences may leave lasting fingerprints on health.

The findings showed that firstborn children were more likely to be diagnosed with neurodevelopmental conditions and allergies, while second-born children had higher odds of certain substance use disorders, migraines, gastrointestinal conditions and infections.

Among the conditions more frequently associated with firstborns were autism, attention-deficit/hyperactivity disorder (ADHD), Tourette's syndrome, obsessive-compulsive disorder, depression, anxiety, asthma and food allergies. Second-born children showed higher odds of substance use disorders, gastritis, duodenitis, shingles, biliary tract disease and kidney infections.

The split was relatively balanced: 79 conditions were more likely among firstborns, while 71 were more likely among second-borns.

WHY BIRTH ORDER COULD MATTER

Rzhetsky's team analysed anonymised insurance claims from approximately 5.1 million two-child families.

Led by first author Benjamin Kramer, the researchers conducted two analyses: one comparing 1.6 million matched sibling pairs across families and another examining siblings within families.

Both approaches produced broadly consistent results. Researchers controlled for factors including parental age, sex and the interval between births.

Siblings offer researchers a useful way to investigate environmental influences because they share parents and much of their early-life environment, while inheriting different combinations of parental genes.

However, birth order itself does not establish the cause of any particular health outcome.

“Physicians may want to watch different populations more closely for different things,” Rzhetsky said, adding that a paediatrician who knows a whole family could tailor screening or preventive advice according to birth order, watching more closely for allergies in a first child, or for signs of gastrointestinal disorders in a second.

One intriguing finding concerned the age gap between siblings.

Shorter intervals between births were associated with a reduced risk of several conditions, including some neuropsychiatric diagnoses among older siblings and allergies and asthma among both children.

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