Mothers who are active during pregnancy end up having children who do more exercise, research shows.
Doing exercise during pregnancy was linked to active children
Children are also more likely to be sporty if they have an autumn birthday, the findings suggest.
The Bristol University-led study of 5,500 11 to 12-year-olds, ruled out any biological factors, the British Medical Journal reported.
Instead, the researchers said active pregnant women were likely to continuing to do exercise after birth.
This meant that they set the children a good example and encouraged their children to get into good habits.
Lead researcher Calum Mattocks said: "The early pre-school years are so important. It seems if they see their parents doing regular exercise this will have a positive impact."
The team gathered data on 11 and 12-year-old children's activity over the course of at least three days, analysing these against several factors including how active mothers were during pregnancy.
They found the children of those that took part in regular brisk walking and swimming while pregnant ending up 3% to 4% more active.
It comes as statistics show child obesity has doubled in the last decade with one in four now obese.
Other factors, such as season of birth, had a marginal impact.
The researchers suggested that children born in the autumn might be motivated to do more sport because they excel at it, as they tend to be the biggest, strongest and most developed in their school year.
Autumn babies who have gone on to enjoy sporting success include Manchester United and England footballer Rooney.
Paul Sacher, founder of the Institute of Child Health's Mend Programme, which works with families on adopting healthier lifestyles, said: "We know children copy the behaviour of their parents.
"This is known as modelling and is very apparent when it comes to exercise and eating."