BEIJING, July 11 (Xinhua) -- A Chinese family planning official has warned against complacency over the country's stable population growth, admitting the growth among some sectors of the population was still unknown.

    "China has succeeded in reducing birthrate since the implementation of family planning policy in the 1970s, but over-optimism pervades in the public, due to misreporting in some regions," Yu Xuejun, spokesman with the State Population and Family Planning Commission (SPFPC).

    In an interview with the central government website to mark World Population Day on July 11, Yu said China had a migrant population of 200 million whose birth situation remained unknown.

    Favorable policies for farming families had spurred a "homecoming" surge among migrant workers and their real birth situation would only be known when local governments ascertained if the returning migrant workers had sired children in cities, Yu explained.

    Each Chinese woman of the child-bearing age had 1.7 to 1.8 children in 2000, but the country would see a baby boom as more only children, products of the government's family planning policy, grew up and married. Couples in which both husband and wife were single children are permitted to have two children.

    China's population is expected to reach 1.36 billion by 2010 and 1.45 billion by 2020. The peak will come in 2033 with a total population of 1.5 billion, according to a report issued by the SPFPC earlier this year.

    Yu said population growth had slowed from the 1970s to 2000, and the birth rate had remained stable from 2000 to 2006.

    Since 2006, the government has focused on solving other population problems, such as gender imbalance and the aging population.

    China's gender ratio for newborns in 2005 was 118 boys for every 100 girls, compared with 110 boys to 100 girls in 2000. In some regions, the figure has reached 130 boys for every 100 girls.

    Yu reiterated that China would not relax its population control policy in the future.