An old poster with a new family planning slogan is found on the Web. XINHUA

Family planning slogans reflected living and production conditions: "Bear fewer children, and plant more trees" or "Raising more children is not as good as raising more pigs".

Some were more ominous, such as "One more baby means one more tomb" and "Uncontrolled births cause houses to collapse".

In 2007, the government softened the tone with new phrases: "For Mother Earth, control childbirth" and "Fewer and healthier births make the countryside prosperous". Some stressed gender equality: "Boys and girls are equally important" and "Girls matter to the nation's future".

Yet all "still strongly reflected the one-child policy", said Zhu, of the CPPCC.

According to official data, the one-child policy was estimated to result in the prevention of 400 million births, and China's population dropped from 22 percent of the global population in the 1970s to 19 percent by 2010.

Outdated content on the issue also appears in textbooks, he said, citing a paragraph in an elementary textbook in Zhejiang province that states: "From 1971 to 1998, the one-child policy resulted in a reduction of population, which saved 7.4 trillion yuan in child-rearing costs, almost equal to China's GDP in 1997."

"It's ridiculous," Zhu said, suggesting that if schools teach the idea that "fewer births can save money", children would wrongly believe they were worthless.