First-borns are often spoilt rotten, but researchers have found one apparent downside to being an older sibling - a higher risk of being overweight or obese.

A study in the Journal of Epidemiology & Community Health has found older sisters weigh more on average than their younger female siblings. 

The paper analysed data from the Swedish birth register, combing through the records of over 13,000 pairs of sisters who would become mothers.

Body weight was measured at birth and at their first prenatal appointment, 10-12 weeks into pregnancy.

While older sister averaged a higher birth weight, their Body Mass Index measured 2.5% higher in the early stages of pregnancy.

They were also 29% more likely to be classed as overweight, and 40% more likely to be obese.

Researchers behind the study think the difference might be explained by a changes in the placenta after a woman's first pregnancy.