Older sisters are more likely to be overweight or obese than their female sibling, research suggests.
The largest study of its kind on women found that birth order may play an important role in determining weight.
The findings back up similar research on men that found older brothers were more likely to be overweight, but were also taller than younger male siblings.
The latest research, published in the Journal of Epidemiology and Community Health, examined data from 13,406 sister pairs (26,812 women in total) in Sweden.
Experts took data from the Swedish Birth Register, which was started in 1973, to examine the weight of sisters who were in the early stages – first 10 to 12 weeks – of pregnancy.
The data was collected from their first antenatal visit and included height, weight, lifestyle and family history.
Analysis of the sister pairs, using data from 1991 to 2009, showed that first-born women had a 2.4 per cent higher body mass index – by 0.57 kilograms – than their second-born sisters.
In addition, first-born sisters had a 29 per cent increased risk of being overweight and a 40 per cent increased risk of being obese.
First-born sisters were also negligibly taller, by an extra 1.2 millimetres.
The experts, from the University of Auckland in New Zealand and Uppsala University in Sweden, also noted a “considerable increase” in BMI over the 18-year period, of 0.11kg per year.
They said it was unclear why older sisters seemed to be heavier, but shrinking family sizes may play a role.
They concluded: “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.”