Firstborn women are more likely to be obese as adults than their second-born sisters, finds the largest study of its kind in women.
"Our study corroborates other studies on men, as we showed that firstborn women have greater BMI and are more likely to be overweight or obese than their second born sisters," said the study by Wayne Cutfield from University of Auckland, New Zealand and colleagues.
"The steady reduction in family size may be a contributing factor to the observed increase in adult body mass index (BMI) worldwide, not only among men, but also among women," the study said.
The research involved 13,406 sister pairs — just under 29,000 participants in total.
Information was obtained from antenatal clinic records from the Swedish National Birth Register over 20 years (1991-2009).
The researchers found that firstborns were nearly 30 percent more likely to be overweight and 40 percent more likely to be obese than their younger siblings were. They were also marginally taller.
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, possibly attributable to the 'resource dilution hypothesis' which holds that there's less to go around as a family grows in size, the researchers said.
The study was published online in the Journal of Epidemiology & Community Health.