Health authorities on Monday said they are updating how they measure children's growth for the first time since 1993 to reflect the fact that today's young people are typically taller and heavier.

New charts are being rolled out to help check how a child's weight, height, body mass index and head circumference compare with those of their peers.

Dr Thomas Chung, from the Department of Health, worked with colleagues from Chinese University and the University of Hong Kong. He said there was a clear trend upwards in the height and weight of children and adolescents in the territory.

"When we compare the new charts, HK2020, with that of the old charts, HK1993, the median height of boys and girls at age 18, they are two centimetres taller than those in the median height of the same age in 1993," he said.

"There may be many factors like nutrition, and improvements in society, so this is expected and seen in other places in the world as well."

Chung also said they compared their current data to data routinely collected by the health department to validate and discover trends, adding the pandemic did not affect their results.

"We collected more than 21,000 data from children and adolescents to construct the new Hong Kong 2020 growth references. These growth charts contain nine centile lines, as compared to the seven centile lines in the old charts," he said.

"The two additional extreme lines help frontline professionals have a more accurate and detailed assessment on the growth of the children, so that they can be referred to other specialists if necessary."

Dr Tony Nelson, a paediatrics specialist from Chinese University, said it was important to check local data instead of relying on global studies.

"In terms of our local data showing that infants are lighter at birth and girls are slightly shorter, I think this just reflects the importance of local data," he said.

"So our local data shows that for the Hong Kong population, it's not like the WHO study, which was from six countries globally and included countries like the Netherlands, and India, and America, and other places. So I think this just reflects the local genetics, the local population here."

The Department of Health will start using the new growth charts in maternal and child health centres in July, and in student health service centres in September.

The Hospital Authority will implement them in phases starting in September, and aims to have them in all its facilities by the end of the year.