BEIJING, Feb. 21 -- Beijing relaxed its birth control policy on Friday, allowing couples to have a second child if either parent is an only child.

An amendment to the Beijing Population and Family Planning Regulations was approved at a meeting of the Standing Committee of the Beijing Municipal People's Congress, the city's legislature.

It is a significant change to family planning policy that has been in effect for more than three decades and part of a plan to raise the fertility rate and ease the financial burden of a rapidly ageing population.

"Beijing's average fertility rate has remained at one birth per woman for the past 18 years, which is much lower than the national average that stands at 1.5 and the replacement rate of 2.1," said Geng Yutian, deputy head of the municipal health and family planning commission.

A working population shrinks when the fertility rate is lower than 2.1, experts say.

An expected baby boom will put more pressure on the mega-city, which has a population of more than 21 million.

About 54,200 more people will be added to the population annually in the first five years after the policy relaxation and 40,000 more will be added every year after then, experts have forecast.

"In the short term, they will pose more pressure on kindergartens and primary schools," said Geng.

As half of births in Beijing are concentrated in urban areas, the policy relaxation will be a strain on urban hospitals, he said.

The city government will provide support by improving hospitals, nurseries and primary schools, and by protecting women's right to maternity leave, said Wang Delin, vice chairman of the Legislative Affairs Committee of the Beijing Municipal People's Congress.

About 1,000 beds will be added in the coming three years in obstetrical departments in Beijing hospitals that can cater for 70,000 expecting mothers, said Zheng Jinpu, a member of the commission.

Beside Beijing, Tianjin Municipality and the provinces of Zhejiang, Jiangxi and Anhui have also changed their policy.

Provincial-level governments in Guangxi, Hubei and Jiangsu have announced their intentions to relax the policy in March. Others, including Hunan, Qinghai and Shanghai, promised changes in the first half of this year.