A study has found that conditions frequently diagnosed can differ depending on birth order, such as being a firstborn or secondborn.

On Aug. 10, local time, Japanese online media outlet Gigazine reported that an analysis of more than 10 million two-sibling cases in U.S. commercial health insurance data found 150 conditions with different diagnosis frequencies between firstborns and secondborns.

The study was conducted by a team led by Benjamin Kramer at the University of Chicago, using insurance claims data from 2003 to 2024. The analysis covered 5,135,006 pairs of two siblings, or 10,270,012 people. The team compared 1,616,881 pairs of firstborns and secondborns from different families with similar characteristics such as sex and birth year. It also separately compared firstborns and secondborns within the same family to reduce the influence of genetic factors and the home environment.

After examining 569 conditions, the researchers found statistically significant differences by birth order in 150 of 418 with enough cases. Of these, 79 conditions were diagnosed more often in firstborns and 71 more often in secondborns. Rather than one side being generally more vulnerable, the differences varied by type of condition.

Specifically, firstborns had about 43 percent more diagnoses of pervasive developmental disorder not otherwise specified (PDD) than secondborns. Autism was about 26 percent higher, food allergies about 20 percent higher and allergic rhinitis about 9 percent higher. By contrast, secondborns had about 35 percent more diagnoses of shingles than firstborns. Substance abuse was about 19 percent higher, gastritis and duodenitis about 14 percent higher, and migraine about 13 percent higher.

As a possible reason allergies were diagnosed more often in firstborns, the study suggested secondborns may be exposed earlier to a wider range of microbes through older siblings. It said such an environment could affect the development of immune tolerance.

It said differences in neurodevelopmental disorders were difficult to explain by a single cause. The team said a combination of factors was likely behind higher diagnosis rates for firstborns in PDD and autism, citing "biological differences by pregnancy order, differences in parental observation, timing of diagnosis, and factors that could not be excluded in the analysis."

The results cannot be taken to mean that firstborns or secondborns are actually more prone to specific diseases. The study analysed diagnoses recorded by medical institutions and insurance claims data, not actual incidence. Cases without medical visits were not included, and it is difficult to fully exclude variables such as care-seeking behaviour, timing of diagnosis and parental age. The team also drew a line, saying birth order itself does not mean it is the cause of disease occurrence.

Another limitation is that the analysis was limited to two-sibling families covered by commercial health insurance. It is difficult to conclude the same pattern would appear among families with more siblings, the uninsured, or those enrolled in public health insurance.

Overall, differences by birth order were not large. The median gap in diagnosis frequency between firstborns and secondborns was about 10 percent. The researchers said birth order could still potentially be used as a weak risk indicator across multiple conditions. They said further studies are needed that consider not only diagnosis records but also actual incidence, living environment and family structure.