High altitude could make newborn babies lighter, according to doctors at a hospital in Northwest China's Qinghai Province.

But they have found that the impact is stronger on people from the Han ethnic group than on Tibetans who adapt more easily to highland anoxia.

The doctors, from the Qinghai Provincial Hospital for Women and Children based in Xining, the capital of the northwestern province, monitored 1,012 babies born at the hospital between August 2002 and August 2003.

"Their parents are from four regions at altitudes ranging from 2,200 to 3,700 metres," said Dr Xu Faliang. "They were all weighed with a standard scale for newborns with precision to 25 grams."

The data the doctors collected show that babies from higher altitudes tend to have lower weight at birth. "In fact, babies' birth weight dropped as the altitudes of their hometowns rose."

This relation between altitude and birth weight is more distinct among the Hans, the most populous ethnic group in China, according to Xu.

"For babies from the same altitude, their ethnicities have a lot to do with their birth weight," he added.

In regions 3,000 metres above sea level, babies of Han people tend to have lower birth weight than those of Tibetans, the research has found. "This is probably because the Tibetans adapt more easily to highland anoxia," said Dr Xu.

Their conclusion seems to echo the research finding of another Xining-based hospital, which suggests highland anoxia also leads to a low birth weight in newborn babies.

"This is because highland anoxia could lead to changes in the placenta and its tomenta and therefore affect the normal function of the placenta," says a thesis by doctors at hospital affiliated to the Qinghai Provincial Medical College.

Earlier researches have proven that the people of the Han ethnic group are more vulnerable to highland anoxia than the Tibetans, who have lived at high altitudes for generations and have a larger lung capacity.