The large number of women of childbearing age in Shanghai and the rise in migrant residents have seen the city's population surge in recent years, experts said on Wednesday.

The city's birth rate has been on the rise since 2009, with 226,100 babies born last year, up by 25.6 percent year-on-year, Shanghai Population and Family Planning Commission announced earlier this week.

Among the newborn babies, 121,100 were to registered residents in Shanghai.

It is the first positive natural growth rate of Shanghai's registered population after a 19-year stretch of negative population growth, according to the commission.

In 2012, the natural growth rate of Shanghai's registered population was 0.26 in every 1,000, compared with minus 0.68 in every 1,000 in 2011.

But the positive natural growth rate of the registered population might be a short-term phenomenon, said Zhou Haiwang, deputy director of population and development studies at Shanghai Academy of Social Sciences.

"Chinese people saw 2012 as the most auspicious year in China's traditional calendar, so many couples in China were trying to ensure their children were born during the Year of the Dragon, which brought about the birth peak," he said.

"It's likely to have a negative natural growth rate (in the registered population) in 2013 again, the Year of the Snake," he said. But if the large number of babies born to migrant workers is taken into consideration the city is still likely to see a rising birth rate.

Despite the government's efforts to encourage Shanghai residents to have babies, the city's registered population has long had a low birth rate and an aging population.

"More women have postponed their plans to have children in big cities like Beijing and Shanghai," said Zhou.

"The last baby boom was in the 1980s. And people who were born during that period now have entered their marriage and childbearing age," Zhou said. "So they have created a peak in births in recent years."

Sex ratio grows

The rise in the migrant population in Shanghai has also contributed to the city's birth peak in recent years, according to Huang Hong, the commission's director.

By 2012, there were 23.8 million residents in the city, about 330,000 more than the previous year. Migrants who have lived in the city for more than six months are counted as residents.

There were 9.6 million migrants in 2012, about 250,000 more than in the previous year. They account for about 40 percent of the city's total population.

Last year, 105,000 babies were born to the migrant population, up by 33.8 percent from 2011.

"The steady rise in the city's population in recent years is partly due to the increasing migrant population and more births," Huang said.

The population density is increasing rapidly in the city's suburban areas and outskirts, where the migrant population is mainly concentrated. The city is taking efforts to have a balanced population distribution.

Huang said the increasing migrant population has not only brought more births to the city but also widened the gap in the city's sex ratio because many migrants who come from rural areas traditionally favor boys than girls.

Shanghai's sex ratio was 112.4 boys to 100 girls in 2012, a drop of 0.7 from the previous year. The ratio for the city's registered population was 103-107 boys to 100 girls, while the ratio among the migrant population was 117.8 to 100, according to the commission.

The commission has announced that the city will carry out a pilot project for the registration of hospital births and set up stations to monitor the sex ratio.

Under the pilot project, the use of identity cards will be required when registering newborn children. It aims to create a more accurate picture of newborns in the city.

Authorities said they will strictly monitor the sex ratio and crack down on illegal clinics that notify parents of the sex of the unborn and carry out abortions.

In addition, people who provide tips about illegal births will be rewarded by the government.

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