Firstborn children end up a little taller, smarter and richer than their younger siblings, on average.
But are the eldest kids more likely to develop diabetes and heart disease when they grow up, too?
That's the suggestion from a study conducted by researchers in New Zealand, who found that firstborn children under age 12 had slightly higher blood pressure and were slightly less sensitive to insulin than their younger siblings. The results were published online in the Journal of Clinical Endocrinology and Metabolism.
Doctors are looking more closely at children for early signs of diabetes and heart disease, as Type 2 diabetes and obesity have become commonplace in children and adults.
So, are firstborns doomed? Not likely.
First of all, this researchers measured the health of 85 children at one point in time. It's what's called an association study; it can't prove that being born first caused the differences the scientists found.
"As far as I know there's not a strong association between insulin sensitivity in an ... 8-year-old and an adult at 40 or 50 or 60," said Tamara Wexler, an endocrinologist in private practice. "They're not relating this to pediatric obesity and pediatric diabetes."
The researchers found that the 32 firstborns had 21 percent less insulin sensitivity when compared to other children. Being less sensitive to insulin, also called insulin resistance, can be a precursor to diabetes. The firstborns also had a small increase — about 4 millimeters of mercury — in blood pressure.
Those differences were similar to what's seen in obese children, the New Zealanders noted. But these children weren't overweight. They were taller and slimmer than their peers, even when the researchers adjusted the results to account for the height and weight of the parents. There's no clear mechanism that explains the odd mixture of height, weight, blood pressure and glucose tolerance, the researchers wrote.
"We do think this is a risk factor," says Wayne Cutfield, a pediatric endocrinologist at the University of Auckland who led the team. "But this does not mean these people will get diabetes."
Cutfield told Shots that this firstborn effect, if it proves out, could be one of many risk factors for developing diabetes in adulthood, including "unfortunate genes from your parents, crappy diet, not exercising, and stress from work."
Studies with large numbers of adults need to be done to see if firstborns really do have more diabetes or other health issues later in life, Cutfield added.
As for the cause, Cutfield speculates that it could be because the placenta is not quite as efficient in delivering nutrients during a first pregnancy. But many other factors could be involved. "It's really not clear," he says.
Earlier studies on birth order and health issues aren't much help. The previous research tends to be more socioeconomic, looking into which children get the best food and medical care. Firstborns usually do well there.
If firstborns were slightly more likely to get diabetes, Cutfield says, that might not make a big difference for individuals. But it could make a big difference for countries like China, which has enforced a one-child policy for decades. As a result, 60 percent of people worldwide are firstborns, according to Cutfield.
"I don't think that this can be taken as a cautionary tale for firstborns at all," Wexler says. All those firstborns needn't panic, she says. "The positive influence of exercise and diet holds the same, regardless of birth order."