Over 28% of five-year-olds in Wales overweight
Over 28% of five-year-olds in Wales are overweight, with 12.5% of children classed as obese, new figures from Public Health Wales show.
Merthyr Tydfil and Rhondda Cynon Taf have the highest number of overweight children, while the Vale of Glamorgan and Monmouthshire have the least.
The problem in Wales is worse than England and Scotland.
Health experts said the figures were "worrying" but children still had time to adopt a healthier lifestyle.
The heights and weights of 29,400 reception age children were collected across Wales in the 2011-12 academic year as part of Public Health Wales' first Child Measurement Programme report.
The results showed that seven out of 10 children aged four to five had a healthy weight but 28% were classed as overweight or obese.
The report said the figures were "significantly higher" than in every region in England, where on average 23% of children were overweight, with 9.5% classed as being obese.
They were also higher than in Scotland, where 21% of children were overweight, of which 9.8% were obese - although children in Scotland were measured close to their sixth birthday.
The report also pointed to a "clear association between deprivation and obesity among four to five-year-olds in Wales".
Dr Ciaran Humphreys, consultant in public health for Public Health Wales, said it was the first time they had been given a clear picture of how children in Wales were growing - and it was a picture that caused concern.
He said change was needed across society.
"This can be anything from making our communities more pedestrian and bicycle friendly to reducing access to unhealthy fast food near schools," he said.
"As with most health risks, the sooner they are tackled the easier they are to address and the greater the long-term benefits."
He added: "Encouraging healthy eating and regular exercise at a young age provides children with an excellent start and helps them grow up to be healthy adults."
In Merthyr Tydfil, 34% of four to five-year-olds were overweight or obese, while in neighbouring Rhondda Cynon Taf, the figure was nearly 32%.
In contrast, in the more affluent areas of Monmouthshire and the Vale of Glamorgan, the figures were 22% and 25% respectively.
Dr Ruth Hussey, the chief medical officer for Wales, said the information on children's growth would help the Welsh government tackle the problem.
"Together, we must work for a healthy, active and resilient community in Wales where all children have the best start in life," she said.
Last month, a group chaired by Paralympic multi-gold medallist Baroness Tanni Grey-Thompson said that PE should be given the same status as maths, English, science and Welsh in schools to help tackle obesity in Wales.
The Welsh government had asked them to look into how schools could increase levels of physical activity in children and young people.
The group said elevating PE to be a core part of the curriculum would mean more time was devoted to it.