Firstborn women are likelier than their second-born sisters to be obese or overweight as adults, according to a study published in the Journal of Epidemiology & Community Health.

Researchers wanted to know if birth order had an effect on the weight and height of adult women, drawing on similar research on males which showed that birth order has an effect on the weight and height of adult men. The study used data from the Swedish Birth Register which contains information on 99 percent of births in Sweden, focusing on those born between 1991 and 2009.

There were 206,510 girls born within the time period as either first- or second-borns to mothers at least 18 years of age at the time of their birth. The researchers managed to gather 13,406 pairs of sisters, bringing the number of participants for the study to just under 29,000. Twins were not included in the study.

Heights and weights were measured and information was collected as well regarding current lifestyles, health status and family history during the first antenatal visit.

Based on the results of the study, firstborns were found to be slightly lighter compared with their second-born sisters at birth. As adults, though, during their first trimester of pregnancy, they had 2.4 percent higher BMIs than their sisters. Additionally, firstborns were also 40 percent likelier to be obese and 29 percent likelier to be overweight.

In terms of height, however, firstborns were marginally taller than their second-born sisters.

This may be due to the "resource dilution hypothesis," which says that, as families grow, there are fewer resources to go around, accounting for why the larger the number of siblings is, the lower the chances are of being tall.

As the study represents observational research, it doesn't offer a definitive conclusion as to why the results are such. However, the findings of the study do back up previous research on male birth order and point as well to growing evidence suggesting firstborns have higher risks of developing health problems like high blood pressure and diabetes later in life.

The researchers do believe, though, that the steady decline of family size around the world may be one of the factors contributing to an increase in the body mass index levels of adults, not only in men but in women as well.

Photo: Jake Stimpson | Flickr