Groundbreaking New Zealand research has found the order in which you were born, may impact your chances of being overweight.

Researchers at Auckland University's Liggins Institute have analysed more than 13 thousand pairs of Swedish sisters and found firstborns are significantly more likely to be overweight or obese than their second-born sisters.

Professor Wayne Cutfield said the findings back up three earlier studies by his team, and collectively show that firstborn men and women are at greater risk.

He thinks that might be related to the fact that blood vessels to the placenta are narrower during a first pregnancy.

"The findings that we see may be a consequence of some subtle nutrient restrictions to the foetus."