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."