First women more likely to be overweight, ibese

While past studies have suggested first born children are smarter. A new research brings some bad news for first born women with a younger sister: they are more likely to be overweight or obese

Firstborn women were more likely to be overweight or obese than their second born sisters, according to the results of a new study.

Previous studies have revealed firstborn men are more likely to be taller and are at greater risk of overweight or obesity than their younger brothers.

However, according to study co-author Prof. Wayne Cutfield, of the University of Auckland in New Zealand, and colleagues, data on whether similar patterns occur among firstborn women are limited - something they believe needed to be addressed.

"This is important as there is well-described sexual dimorphism in association with early life events, with contrasting effects on long-term health and disease observed in men and women," note the authors.

As such, the team set out to determine whether birth order influences women's height and weight in early life and adulthood. Firstborn sisters at 40% greater obesity risk than secondborn sisters

The researchers analyzed data from the Swedish Birth Register, assessing 303,301 women who were born between 1973 and 1988 - 206,510 of whom were firstborn or secondborn - who gave birth between 1991 and 2009.

Fast facts about obesity • Around 78.6 million American adults are obese - around a third of the US population • The estimated annual cost of obesity in the US stands at around $147 billion • Obesity raises the risk of health conditions such as heart disease , stroke and type 2 diabetes .

In total, the team identified 13,406 pairs of sisters (26,812 participants). The team says they wanted to assess sisters so they could account for any shared or genetic early-life influences.

The sisters' weight and height were assessed at birth and at their first prenatal visit when they were around 3 months pregnant. Information about sisters' health, lifestyle and family history was also gathered at the first prenatal visit. While firstborn sisters were found to be lighter at birth than their secondborn sisters, their body mass index ( BMI) was around 2.4% higher during their first 3 months of pregnancy. In adulthood, firstborn sisters were around 20% more likely to be overweight and 40% more likely to be obese than their secondborn sisters, according to the study findings. In addition, the team found firstborn sisters were an average of 1.2 mm taller than secondborn sisters in adulthood.

Sisters with a higher number of siblings were more likely to be shorter, which the researchers say may be down to the "resource dilution hypothesis" - the idea that parental resources are spread more thinly the more children they have.

The number of children in a family, however, was not associated with the risk of overweight or obesity, according to the authors.