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A new study suggests that women who are the firstborn in their families tend to carry more weight than their younger sisters, Today reports.

The study, which was published in The Journal of Epidemiology & Community Health, was based on data collected from the Swedish Birth Register that dates back to 1973. The database contains information collected about Swedish women at the time of their first prenatal doctor’s visit—including their height, weight lifestyle factors and family histories.

The study was co-authored by Wayne Cutfield, a professor of pediatric endocrinology at the Liggins Institute in New Zealand, and specifically focuses on pairs of sisters who were at least 18 years old at the time of their first visit.

Researchers believe that the conditions of a woman’s uterus changes after she gives birth to her first child. According to Cutfield, during the first pregnancy, blood vessels to the placenta tend to be more narrow.

“And this information has led to the hypothesis that firstborns were exposed to in utero compromise, which reprograms metabolism and the regulation of fat,” he explains.

Tamara South tells Today that she’s not the least bit surprised by the study. For years, South and her older sister—who is 15 years her senior—have tried to figure out why their bodies are so different.

“I was always jealous of her big breasts and hips. And she always wanted to look like me,” South confessed.

“I have to pack on the carbs, and she has to get rid of them,” said South’s 34-year-old sister, who is a stay-at-home mom.

The author of the study notes that the differences in weight between siblings have nothing to do with genes but speak specifically to the baby’s environment during gestation. Of course, two major causes of obesity are that people are overeating and failing to exercise; however, the tendency firstborns have to pack on the pounds paired with the fact that families are producing fewer children could be a possible link to the increasing percentage of people who are overweight. According to Cutfield, because couples are having fewer children, a larger percentage of the population are firstborns.

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