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 per cent more likely to be overweight and 40 per cent more likely to be obese² 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 1lb 4oz heavier on average than second-born sisters. This meant their body mass index (BMI) was 2.4 per cent higher. Firstborn sisters were only negligibly taller, measuring an additional 1.2mm on average. 

 

The researchers also noted³ a considerable increase in average weight over the 18-year period, rising by four ounces per year. 

 

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 obesity⁴ figures appear to be soaring. 

 

The scientists also said there is mounting evidence that firstborns are more at risk of health problems such as diabetes⁵ and high blood pressure in later life than their siblings. But the underlying⁶ causes for these differences are far from clear, they added.

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