China's top health authority is closely monitoring population fluctuations and will intensify population research, a government spokesperson said on Thursday.

The universal second-child policy, adopted in 2016, has for the last two years influenced the number of births in China, with the number of second children accounting for around half the total children born each year, Song Shuli, spokesperson for the National Health Commission, said at a press conference.

Birth is affected by a variety of factors in China, including the number of women of childbearing age, the average age of women getting married and giving birth, and social and economic factors, Song said.

The total number of births in 2018 will be released soon, she said.

The number of births in China saw a slight drop in 2017, the second year of the second-child policy, to 17.23 million, according to the National Bureau of Statistics, which surprised many experts.

The policy allows all couples in China to have two children, and was introduced to counter the negative effects of an aging population and a dwindling workforce in China.

A number of population experts have predicted the number of births will continue to fall for 2018, to between 15 and 16 million, a drop of more than 1 million year-on-year.

Tao Tao, a population researcher at Renmin University of China, said a major reason for the declining birth rate in China is the decreasing number of women of childbearing age.

"The number of women between 15 and 49 in China has been falling since 2011, the year when it reached a peak," she said.

The average ages of women bearing their first child and second children have both increased by one year over the past three years, which has also affected population figures, she said.

Social and economic policies related to birth should be improved to encourage couples to have a second child, she said.