What can you birth order say about your weight? According to a new study, first-born women are more likely to be overweight or obese than their younger sisters.

There’s a lot of research on the relationship between birth order, personality and health. It’s been linked with intelligence, political ideology and cardiovascular disease. For obvious reasons, many don’t hold up to further scrutiny and it’s probably best to take these findings with a pinch of salt. But what’s particularly interesting about this recent study is that it’s the largest of its kind and builds on previous research that reported similar findings.

The study, published in the Journal of Epidemiology & Community Health, collected a range of data from the Swedish Birth Register, which included the weight and height of nearly 13,500 pairs of sisters. Researchers note that all large studies examining the effects of birth order on someone’s BMI – or body mass index – have previously been carried out on men, which prompted them to investigate whether there were similar patterns among women.

Researchers studied sibling pairs born between 1991 and 2009 to women who were at least 18 years old at the time of their first pregnancy. Twins were not included in the study.

They found that at birth first-born women weighed less than their siblings, but were more likely to be overweight or obese when they grew up. First-born women were 29% more likely to be overweight and 40% more likely to be obese, when compared with their second-born sisters. Also, the average BMI for first-borns was 2.4% greater than their second-born sisters.

“Our study corroborates other large studies on men, as we showed that firstborn women have greater BMI and are more likely to be overweight or obese than their second born sisters,” researchers note in the study.

“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,” they add.

It’s unclear why first-born women were more likely to be overweight. A co-author on the paper, Professor Wayne Cutfield from the Liggins Institute at the University of Auckland, speculates that this pattern could be partly down to a change in the amount of blood the placenta receives between first and later pregnancies. He told CBS News that blood vessels may be narrower in the first pregnancy, which could in turn reduce the nutrient supply; changing how fat and glucose is regulated in the body. First-born women could therefore at risk of storing more fat and have less effective insulin.

Researchers were, however, quick to point out this is an observational study and therefore no definitive conclusions can be drawn about cause and effect. They also warn that although the research suggests birth order could be a risk factor for obesity, it’s clearly one of many and is probably only a small contributor.