Urban couples are only allowed one child while rural couples are permitted two in the world's most populous country, under rules introduced in the late 1970s.
Mr Zhang warns that well-off middle class couples benefiting from China's booming economy are increasingly prepared to pay the fines for exceeding their quota.
He also believes rural couples are getting married earlier than the legally-prescribed minimum marriage ages. Women cannot marry before their 20th birthday while men must wait until they are 22.
"Early marriages are still prevailing in some parts of the country, especially rural areas, which goes against the family planning policy," he said.
China's birth rate stands at 13.45 births per 1,000 members of the population, according to US government estimates. This represents a population growth rate of 0.6 per cent for a country already home to one-fifth of the global population.
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