A Swedish study found that birth order and the number of siblings you have may affect your risk of heart disease and mortality. The study followed 2.68 million people in Sweden from 1990 to 2015 and found that first-born men and women with one or two younger siblings had a lower risk of death and nonfatal cardiovascular events than people without siblings.

According to WebMD, if the eldest had more than two younger siblings, that benefit diminished and those participants had a slightly increased risk of cardiovascular disease. The study was published in BMJ Open last month. One of the study authors, Dr. Peter M. Nilsson, associate professor in the Department of Clinical Sciences at Lund University in Sweden, explained that the findings may have less to do with genetics than family history, social ties, and early influences in life.

He said that parents may focus their attention more on a first-born child before the younger siblings arrive, giving them a healthier start in life.

“The parents try to supervise everything, but the third and fourth born have less supervision, so they might tend to smoke more, drink more than the first born,” Nilsson said, according to WebMD, adding that these lifestyle behaviors affect longevity and cardiovascular health.

Among the 1.32 million women who participated in the study, the eldest in a family compared to only children had slightly reduced mortality and heart disease risk. However, these women had a 17% increased risk of heart disease if they had two or more younger siblings. Their risk of suffering a heart attack increased by 3% if they had four or more siblings, according to WebMD.

Mortality risk decreased by 9% for the first-born children among the1.36 million men in the study versus the men who had no siblings. But the men who had at least four younger siblings had an up to 10% increased risk for heart disease, compared to men with one to three brothers or sisters, who had a slightly reduced risk.

Nilsson and his colleagues also found that the risk for coronary heart disease increased by up to 23% among fifth-born men, and 22% among fifth-born women, showing how birth order can affect cardiovascular disease. While the study did not explain the mechanism for these variables in cardiovascular and mortality risk factors, it does provide an interesting landscape for future research.

“Cardiovascular disease is the biggest cause of death in the world, so when you find these discrepancies between all-cause mortality and cardiovascular disease, it gives you pause,” said Dr. Ethan Weiss, a preventive cardiologist at the University of California, San Francisco, who was not involved in the study.

Since other studies have found that first-born children are at higher risk for cardiovascular and metabolic disorders, Weiss said he would like to have the Swedish study repeated.

Nilsson said that while conventional risk factors for heart disease  — smoking, obesity, and drinking too much alcohol — are obviously important, his study reveals that family history and structure may play a role in determining cardiovascular health and mortality risk factors.