China's top family planning body has warned of a "population rebound" as couples flout one child policy rules.
Chinese couples are only meant to have one child
The widening wealth gap could lead to a rise in birth rates, Zhang Weiqing, from the National Population and Family Planning Commission, told state media.
Newly rich couples can afford to pay fines to have more than one child, while rural couples are marrying earlier, he told Xinhua news agency.
China has about 1.3 billion people, 20% of the world's total.
It is keen to curb its population growth, and the controversial family planning policy, implemented in the late 1970s, is meant to limit urban couples to one child and rural families to two.
But rising incomes mean that some newly rich couples in urban areas can easily afford to break the rules and pay the resulting fines.
In fact, last month, a survey by the National Population and Family Planning Commission found that the number of rich people and celebrities having more than one child was on a rapid increase, and nearly 10% of people in this category had three children.
In the countryside, too, the rules are being flouted.
China's constitution says that men may marry at 22 and women at 20, with late marriages and later childbearing encouraged.
But according to Mr Zhang, "early marriages are still prevailing in some parts of the country, especially in rural areas, which goes against the family planning policy".
Part of the reason why rural families refuse to comply is because of the traditional preference for sons.
Experts say this preference has led to the under-reporting of female births, as well as abortion of female foetuses and female infanticide.
By the end of 2006, China's population stood at 1,314,480,000, according to the National Bureau of Statistics, with males accounting for 51.5% of the population.