A study by experts from the University of Auckland in New Zealand and Uppsala University in Sweden suggests that birth order may play an important role in determining weight.
The research found that younger sisters are more likely to be slimmer than firstborn girls.
The researchers looked at data on the health of 13,400 pairs of sisters and found that firstborn women were, on average, 29 percent more likely to be overweight and 40 percent more likely to be obese than second-born sisters.
The researchers believe there is mounting evidence that firstborns are more at risk of health problems such as diabetes and high blood pressure in later life than their siblings.
The findings of the researchers back up similar research on men that found firstborn males were more likely to be overweight than younger brothers.
The research was published in the Journal of Epidemiology and Community Health.
S.O.Z