CHICAGO, Ill. (WOWO) — Birth order may be associated with certain health conditions, according to a massive study examining medical records involving about 10 million American siblings over two decades.

Researchers from the University of Chicago and Columbia University analyzed 569 disease categories across families and between siblings, identifying 150 conditions that showed statistically significant associations with birth order.

The findings suggest first-born and second-born children can show different patterns of health risks, although researchers emphasize that the study does not establish that birth order directly causes any particular disease.

First-born children in the study were more likely to have several neurodevelopmental and psychiatric diagnoses, including autism, ADHD, Tourette’s syndrome and obsessive-compulsive disorder. They also showed higher rates of some immune-related and allergic conditions, including food allergies.

Second-born children, meanwhile, showed higher statistical rates of migraines, shingles, substance use disorders and gastrointestinal conditions such as gastritis.

The research was published in the journal Nature Health.

The sheer size of the study gives researchers an unusually large population in which to look for patterns among siblings.

Fox News senior medical analyst Dr. Marc Siegel said the findings raise important questions about why health patterns might differ between children in the same family.

“I love this study because it opens a lot of questions more than it gives answers,” Siegel said.

He offered several possible explanations, while making clear that those explanations are theories rather than conclusions established by the research.

For the higher rate of ADHD among first-born children, Siegel suggested that differences in parental behavior could play a role.

“My theory, they get more stimuli,” he said, describing parents as potentially being more closely focused on their first child.

He also suggested that differences in early environmental exposure could potentially influence the development of allergies.

“I think it’s because the firstborn’s immune system hasn’t seen the environment before,” Siegel said.

The study also found associations involving second-born children and conditions such as substance use disorders and migraines.

Researchers and medical experts caution, however, against interpreting those statistical associations as predictions for individual children.

A child’s position in the family is only one potential factor among many that can influence health throughout life.

The researchers noted that the study relied on commercial health insurance claims. Those records can be affected by factors beyond underlying biology, including whether families seek medical care, how often children are evaluated and diagnosed, and differences in parental attention.

Other factors also can vary between siblings.

Parents may be older when a younger child is born, pregnancies can involve different biological circumstances, and the amount of time between births can vary substantially from family to family.

Those variables make it difficult to determine whether birth order itself is responsible for any of the observed differences.

Siegel also discussed the possibility that biological changes occurring between pregnancies could contribute to differences between siblings, although that remains a hypothesis rather than a demonstrated explanation for the study’s findings.

The researchers ultimately characterize birth order as a modest population-level marker of risk, rather than a tool that can determine whether an individual child will develop a particular condition.

In other words, being a first-born or second-born child does not mean a child is destined to develop one of the conditions identified in the research.

Instead, the study identifies patterns that may help researchers investigate how family dynamics, biology, environmental exposure and other factors interact over the course of childhood and later life.

With 10 million siblings included in the analysis, the research provides a broad look at those patterns — but the authors say substantially more work will be needed to determine what, if anything, causes the differences.