Older sisters often moan that their younger siblings have all the luck. But it seems they really do have something to complain about-for research has found that younger sisters are more likely to be slimmer than firstborn girls.

The study-the largest of its kind-suggests that birth order may play an important role in determining weight. And the researchers warned that firstborns could be at risk of other health problems too. The scientists 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 findings back up similar research on men that found firstborn males were more likely to be overweight than younger brothers.

The latest research examined data from pregnant Swedish women， gathered between 1991 and 2009， Weighed when they were between ten and 12 weeks pregnant， firstborn women were 11b 40z heavier on average than second-born sisters. This meant their body mass index （BMI） was 2.4 percent higher. Firstborn sisters were only negligibly[極小地] taller， measuring an additional l.2mm on average. The researchers also noted a considerable increase in average weight overt he 18-year period， rising by four ounces peryear.

The experts， from the University of Auckland in New Zealand and Uppsala University in Sweden， said it was unclear why older sisters seemed to be heavier. But they said the findings could explain why obesity figures appear to be soaring. The scientists also said 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. But the underlying causes for these differences are far from clear， they added. Writing in the Journa/ of Epidemio/ogy and Community Hea/th， the team said： “Our study corroborates other large studies on men， as we showed firstborn women have greater BMI and are more likely to be overweight or obese than their second-born sisters. 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.”