Older siblings who complain that the youngest child always gets the best genes may be on to something.
New research, published online in the Journal of Epidemiology and Community Health, found that younger sisters tend to be slimmer than their firstborn sisters. The study used the Swedish Birth Register to gather data from women who were pregnant between the years of 1991-2009. The study suggest that the order of birth among siblings has an affect on weight, body mass index, and risk for other health problems.
Analyzing the health information of 13,400 pairs of sisters, scientists discovered that firstborns were 29% more likely to be overweight and 40% more likely to be obese when compared to their younger sisters. Similar results were found among men who had younger brothers. The researchers write:
“Our study corroborates other large studies on men, as we showed that first born women have greater BMI and are more likely to be overweight or obese than their second born sisters.”
Though firstborn sisters were lighter at birth, their body mass index (BMI) was found to be 2.4% higher during their first trimester compared to the BMI of their second-born sisters during their first three months of pregnancy
The team from the University of Auckland in New Zealand and Uppsala University in Sweden are unsure why firstborn sisters tend to weigh more, but they believe it offers an explanation for why obesity rates are at an all-time high. As family sizes are dwindling, more parents are having fewer children and so increasing the number of firstborns.
The study’s findings add to a growing amount of research that indicates firstborns face greater risk for health problems, including diabetes and high blood pressure.