The surprising trait eldest children are likely to have.

A new study has shown that firstborns are nearly 30 per cent more likely to be overweight as adults. It also shows that they’re 40 per cent more likely to be obese than their younger siblings.

Researchers focused on pairs of sisters who were both over 18 years at the time of the study.

The study suggests this tendency towards a higher weight has nothing to do with genetics.

It is not known exactly why, but it is thought that during a mother’s first pregnancy her blood vessels narrow and after her first baby a woman’s uterus changes.

After her first born it's thought that a woman's uterus changes.

"This information has led to the hypothesis that firstborns were exposed to in utero compromise, which reprograms metabolism and the regulation of fat," Wayne Cutfield, co-author of the study and a professor of peadiatric endocrinology at Liggins Institute in New Zealand told

"The fact that families are having fewer children, could be an unexpected link to the growing percentage of people who are overweight or obese," Cutfield told "Because many people are having fewer children, a larger proportion of the population will be firstborn," he explained to the US publication.

An interesting finding and one that the researchers believe may be a good insight in to why so many people are overweight today.

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