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Older sisters are more likely than younger sisters to develop obesity in adulthood, according to a new study.

The latest research, which involved 13,406 sister pairs born in Sweden between 1973 and 1988, reveals that firstborn women are significantly more likely to be overweight or obese as adults than their second-born sisters.

While firstborns are more likely to be be lighter than their second born sisters as newborns, they were 29 percent more likely to be overweight and 40 percent more likely to be obese compared to their second born sisters.

"Our study corroborates other large studies on men, as we showed that firstborn women have greater BMI and are more likely to be overweight or obese than their second born sisters," researcher wrote, according to a statement.

 "The steady reduction in family size may be a contributing factor to the observed increase in adult BMI worldwide, not only among men, but also among women," they concluded.

The latest findings were published in the Journal of Epidemiology & Community Health.