Having a younger sister is the best thing as you can play together, share each other clothes and talk about everything to her.

But what if having a younger sister causes you a disadvantage like becoming overweight?

You would be surprised to know that having a younger sister increases your chances of becoming overweight, and here’s why.

According to a study that was done on 13,406 pairs of Swedish sisters born between 1991 and 2009 that concluded first-borns have a greater body mass index (BMI) and are more likely to be overweight or obese.

According to the study, older sisters were slightly lighter than their younger sisters at birth but when they grew up and expected children themselves, they had a 2.4% higher BMI.

They discovered that sisters who were born earlier are 29% more likely to be overweight than their younger sisters. They are also 40% more likely to be obese.

Another theory is cultural. Dr Maria Pena, Director of the Center for Weight Management at Lenox Hill Hospital in New York City, remarks that mothers are more likely to overfeed their earlier babies than their later ones in an attempt to ensure a “healthy weight.” This eating habit could stick throughout their lives.

Sibling rivalry is yet another plausible theory. “Maybe the first-born competes with the second-born for food in the household. Maybe more money gets spent on the firstborn,” theorizes Gary Sacks of Deakin University in Melbourne, Australia who thinks that it would be interesting to examine possible social causes.

Professor Wayne Cutfield from the Liggins Institute at the University of Auckland says that the blood vessels that supply nutrients to the fetus seem to be slightly thinner in cases of first pregnancies.

As a result, there is a possible reduction of nutrient supply, causing the individual to be at risk of storing more fat and having insulin that works less effectively later in life.