Research suggests that girls are nearly 50% more likely to be overweight than boys by the time they reach the age of seven.
According to a study by the Institute of Education, nearly one in four (23%) girls weighs too much at this age, whereas just over one in six (18%) boys is too heavy.
The weights of 11,000 children included in the Millennium Cohort Study - which is tracking children born between 2000 and 2002 - were analysed by researchers.
In addition to gender, the figures reveal that the number of brothers and sisters a child has is also an indicator of obesity.
Seven year olds who have no brothers or sisters are 25% more likely to be overweight than those with one sibling and 30% more likely to be overweight than those with two.
The study also found that youngsters aged five who are overweight are 25 times more likely than children of normal weight to still be too heavy two years later.
Youngsters with an overweight or obese mother, obese father or a mother who smokes are also likely to be heavier.
Principal author Dr Alice Sullivan said: “Girls and only children are more likely to become overweight between the ages of five and seven.
“It is not clear whether the increased risk for girls is due to them being overfed compared to boys, or because they are involved in less physical activity - perhaps due to the over-protectiveness of parents - or some combination of the two.”