- New research suggests that your birth order may impact your disease risk.
- There are a few theories on what could be behind this.
- Doctors say your odds of developing a disease are impacted more by other factors.
There are a host of things that can influence your risk of developing a disease, from genetics to environmental exposures to lifestyle factors. But new research suggests there’s one more that probably isn’t on your radar: your place in your family’s birth order.
The study, published in Nature Health, analyzed health data from more than five million two-child families—and found a clear link between an individual’s birth order and how likely they were to develop certain diseases. While first-borns in the study had a higher risk of neurodevelopmental conditions like ADHD and immune-allergic diseases like food allergies, second-borns were at greater risk for gastrointestinal disorders and substance abuse disorders. Another interesting finding: A shorter gap between siblings’ ages seemed to protect against neuropsychiatric and allergic conditions.
Andrey Rzhetsky, Ph.D., study co-author and professor of medicine at The University of Chicago Biological Sciences Division, says he decided to do the study after observing several families in which older and younger siblings had different health issues and temperaments.
While Rzhetsky’s study didn’t explore why these differences exist, doctors say there could be something here. Below, experts break it down.
Meet the experts: Andrey Rzhetsky, Ph.D., study co-author and professor of medicine at The University of Chicago Biological Sciences Division; Mark Hicar, M.D., Ph.D., chief of pediatric infectious diseases at the University at Buffalo Jacobs School of Medicine and Biomedical Sciences; Danelle Fisher, M.D., pediatrician at Providence Saint John’s Health Center; Ashanti Woods, M.D., pediatrician at Mercy Medical Center.
What disease risks does birth order carry?
The study compared each child’s odds of being diagnosed with a disease against their sibling’s odds of developing the same disease. After crunching the data, the researchers found that first-borns and second-borns did have relatively higher odds of developing some conditions.
What the data showed for first-borns:
- Autism, 26% higher risk
- Attention-deficit/hyperactivity disorder (ADHD), 7% higher risk
- Food allergies, 20% higher risk
- Hay fever, 9% higher risk
What the data showed for second-borns:
- Substance abuse, 19% higher risk
- Gastrointestinal conditions, 14% higher risk
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What’s behind this?
The study didn’t look at why birth order affected the risk of developing some diseases. However, there are a few theories.
“It is possible there is something inherent to the order,” says Mark Hicar, M.D., Ph.D., chief of pediatric infectious diseases at the University at Buffalo Jacobs School of Medicine and Biomedical Sciences in Buffalo, New York. “There are biological differences between the first pregnancy and subsequent pregnancies, including age, changes to mom’s body, and differences in immune responses.” Each baby technically contains foreign material from the father that the mother’s immune system must become tolerant to, which “has been shown to improve with subsequent pregnancies,” Dr. Hicar adds.
Rzhetsky points out that siblings also tend to share a microbiome. “This could explain signals in infectious diseases, allergies, and the potential protective effect of a small age gap for neurodevelopmental conditions,” he says.
It also could have something to do with parental behavior. First-time parents may be more likely to escalate a concern to a healthcare provider, prompting an evaluation and diagnosis, compared to when they have more children, Rzhetsky says. The parents’ age may matter, too. “Structurally, birth order correlates with parental age at birth, meaning first-borns inherently have younger parents,” he says.
Dr. Hicar agrees that how parents treat first-borns and second-borns may have an effect. “If your first-born has a neurological condition, you may get more testing with subsequent pregnancies,” he says. “Another example is that planned environmental and dietary exposures tend to be stricter with first-borns than later pregnancies as well.”
Ashanti Woods, M.D., a pediatrician at Mercy Medical Center in Baltimore, stresses that, at least for now, there are no clear answers. “As a pediatrician, I welcome such research to hopefully one day be able to thoughtfully answer, ‘What caused my baby’s …’ questions from parents,” he says. “With that said, with any research study, it is important to delineate correlation from causation. In other words, is this truly a cause of this condition, or simply a pattern observed?”
What does this mean for your actual disease risk?
That’s tricky. The study looked at odds ratios, which is how likely you are to develop a disease compared to your sibling. But that doesn’t mean that someone will actually develop a disease based on birth order, or that their real-world risk is all that high—just that it’s relatively elevated, as compared to their sibling’s risk. “We are talking about population effects. At the individual level, the effects are subtle,” Rzhetsky says.
Diseases are also complicated, points out Danelle Fisher, M.D., a pediatrician at Providence Saint John’s Health Center in Santa Monica, California. “There are a lot of factors that go into many of these diseases,” she says. “Birth order may be possibly one in a number of factors that could influence disease rates.”
Rzhetsky agrees. He points out that things like a person’s sex, having a sibling with the same diagnosis, or an older mother at birth are often stronger signals that someone will develop a certain disease. “Birth order is a weaker marker than nearly anything else already on the intake form, and it is non-modifiable, so it warrants no action,” he says.
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