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First-born women more prone to adult obesity than younger sisters

Older sisters 40% more likely to be obese

Adrian O'Dowd

Thursday, 27 August 2015

First-born women appear to be more likely to be overweight/obese as adults than their second-born sisters, concludes a study* published today in the Journal of Epidemiology & Community Health.

The findings confirm similar research on the impact of male birth order and have prompted the New Zealand researchers to argue that shrinking family size might be playing a part in rising adult body mass index (BMI) globally.

There has been a steady decline in birth rates worldwide, especially in Europe and many Asian countries.

Previous research has shown that first-borns often appear to be taller than their siblings and in adulthood, first-born men are also heavier and of greater BMI.

However, much of this research has been carried out on men and similar data in adult women is scarce.

Researchers from the University of Auckland, therefore, wanted to find out if birth order affected adult women’s height and weight as it appears to among adult men.

They studied data from the Swedish Birth Register, which started in 1973, and which contains information dating back to the first antenatal visit on virtually all (99%) births in Sweden.

They focused on the period 1991-2009 for women who were at least 18 years old at the time of their first pregnancy, and who had been born to a mother who was similarly at least 18 years old at the time.

Weight and height were measured and information collected on current health, lifestyle, and family history at the first antenatal visit.

The results showed there were 303,301 Swedish females born in 1973–1988 who gave birth to a child in 1991–2009. Of the children born, 206,510 were first or second-born. For these, complete data were available for 13,406 sister pairs.

The researchers found that at birth, first-borns were very slightly lighter than their second-born sisters, but as adults during their first three months of pregnancy, their BMI was marginally higher (2.4%) than that of their second-born sisters.

They were also 29% more likely to be overweight and 40% more likely to be obese than their second-born sisters. They were also slightly taller.

Analysis showed that the number of children in a family was not associated with BMI or the odds of being overweight/obese, but having more siblings was associated with shorter height and lower odds of being tall, they said.

This was an observational study so no definitive conclusions could be drawn about cause and effect, and only young women were included in the study.

Nevertheless, the authors said their findings backed up similar research in adult male first-borns.

There was mounting evidence that suggested that first-borns could be more at risk of health problems, such as diabetes and high blood pressure, in later life than their siblings, they said, although the potential underlying triggers for these differences were uncertain.

They concluded: “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.

“The steady reduction in family size may be a contributing factor to the observed increase in adult BMI worldwide, not only among men, but also among women.”

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