Birth order lends itself to many stereotypes. The firstborn is responsible; the middle child is easy-going; the baby is the rebel. Other studies try to pin personality differences on birth order. But a recent study claims that birth order may have an effect on a woman’s weight.

A study published in the Journal of Epidemiology & Community Health recently stated that firstborn women are slightly more likely to be overweight or obese as adults as compared to their younger sisters.

The research focused on data on 13,406 pairs of sisters, amounting to under 29,000 women. The study looked at women who were at least 18 at the time of their first pregnancy and who had been born to a mother at least 18 years old at the time. Their weight and height were measured and information on current health, lifestyle and family history were collected.

The analysis showed that firstborn girls were slightly lighter than their second-born sisters at birth. By the time they reached adulthood, the tables turned. When they were expecting children themselves, firstborns tended to have a 2.4 per cent higher body mass index (BMI) than their second-born sisters. The research also showed that they were prone to obesity or being overweight.

But why are firstborns at greater risk when it comes to certain health issues?

Authors of the study hypothesis that there may be a change in the blood supply to the placenta between the first and later pregnancies, with the vessels more narrow in the first pregnancy. This could reduce the nutrient supply, and re-program the regulation of fat and glucose. This puts the woman at risk of storing more fat and having insulin that works less effectively.

Some experts say other factors may also be responsible. Most first time moms are overtly careful with firstborns. They want the baby to have a healthy weight and feed regularly. With second children, mothers have been there and done that. So they don’t overdo things. The eating habits formed at young ages tend to stay a lifetime. Firstborn women may simply not be listening to the signal in their brain that they need to stop eating, experts believe.

The findings of this study are similar to studies conducted on men. A study by the same research team had found that firstborn men are at an increased risk of diabetes and high blood pressure.