Two years ago, Madam Doris Yeo, managing director of Shanghai School Uniforms, had to disguise her shock when a 13-year-old boy walked into her shop looking to get a uniform with a 59-inch (149.8-cm) waistline - a size typically used by obese adults.

With such requests becoming more commonplace, school uniform manufacturers have extended their ready-made size range upwards to cope with the burgeoning waistlines of overweight schoolchildren.

Madam Yeo, whose company has been in the business for over 50 years, said: "Our children are getting bigger. We have to make bigger sizes now compared to the past, especially for the shorts and trousers."

Shanghai School Uniforms now carries an extensive 12 sizes - up from 10 sizes - for boys' bottoms.

Primary school shorts run from a 20-inch waistline to 42 inches, while secondary school trousers are from 24 to 46 inches. There are plans to go beyond even the biggest sizes it currently offers "to avoid the time needed to do custom-made uniforms", which cost parents about 30 per cent more.

Larger uniform sizes are perhaps a consequence of the rising obesity rates among schoolchildren, which have risen in recent years from 10 per cent in 2000 to 12 per cent in 2014.

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A recent Health Promotion Board study found that if a child is overweight at age seven, he has a 70 per cent chance of growing up into an overweight or obese adult.

Prime Minister Lee Hsien Loong, in his National Day Rally speech last month, also advised Singaporeans to get their health checked, exercise more and watch what they eat, so as to reduce their diabetes risk.

Several other uniform manufacturers also said they now offer a dozen or more sizes, from eight to 10 sizes a decade back.

Ten years ago, a typical Primary 1 boy would easily fall between sizes 20 and 24. But now, he can be between sizes 22 and 26.

Meanwhile, a regular Secondary 1 student would be around 24 to 28 inches in the past, compared to 26 to 32 inches now.

Mrs Helen Quek, executive director of uniform supplier Bibi & Baba, which supplies to more than 50 schools, said sizes for secondary school students, for instance, used to stop at 38 inches, but can go up to 44 inches now.

Madam Yeo Kwee Huay, director of school uniform firm Asencio, which has been in business for 30 years and serves more than 40 schools, said her sizes for primary school pupils can go up to 42 inches, while those for secondary school stop at 46 inches.

Asencio is among the few remaining suppliers that make uniforms locally - despite higher manpower costs and a retiring pool of local seamstresses - so that children who cannot find their sizes do not have to wait up to a month to get their clothes.

"If there is a shortage of uniforms in a particular size and a student needs it fast, there needs to be an in-house production plant here," said Asencio's Madam Yeo, adding that this allows kids who need a bigger size to get their custom-made uniforms within days.

"Overseas partners need the volume, and they might not have the speed to supply uniforms fast enough before school reopens."

Parents have to pay more if their children need off-the-rack uniforms of larger sizes.

On average, uniforms cost between $60 and $80, for two sets of primary school uniform and two sets of PE attire, while those for secondary school are around $100.

Parents said the wider range of ready-made uniform sizes can help parents save considerable time and money, since custom-made sizes cost about 30 per cent more.

Housewife Josephine Yeo, 41, said she has had to spend $80 to buy two sets of ready-made uniforms and PE attire for her 10-year-old son, who is overweight at 41kg. "Now we make him go for runs every few days," she added. "We also try to control his diet. For example, we don't give him as much rice during dinner."

To beat the flab, schools have been encouraging kids to get active, with some unlocking their athletic equipment cupboards and facilities during recess to encourage young people to pick up a ball or a racquet.

Schools also offer parents tips on how to manage their children's weight.

All mainstream schools also offer healthier food under the Healthy Meals in Schools Programme, launched in 2011 to foster better eating habits in the young.

Mr Goh Boon Tiong, senior teacher for physical education at Zhenghua Primary, said there is "a high tendency" for pupils to gain weight during the long school holidays.

Observers said the shift in sizes over the years may be due to changes in affluence and nutrition.

National University of Singapore economics lecturer Kelvin Seah said: "As parents become more affluent, they are able to provide their children with more nutritious food. So this has led to some kids getting bigger, sometimes too big for their own good."