SHANGHAI — Wang Weijia and her husband grew up surrounded by propaganda posters lecturing them that “Mother Earth is too tired to sustain more children” and “One more baby means one more tomb.”

They learned the lesson so well that when Shanghai government officials, alarmed by their city’s low birthrate and aging population, abruptly changed course this summer and began encouraging young couples to have more than one child, their reaction was instant and firm: No way.

“We have already given all our time and energy for just one child. We have none left for a second,” said Wang, 31, a human resources administrator with an 8-month-old son.

More than 30 years after China’s one-child policy was introduced, creating two generations of notoriously chubby, spoiled only children affectionately nicknamed “little emperors,” a population crisis is looming in the country.

The average birthrate has plummeted to 1.8 children per couple as compared with six when the policy went into effect, according to the U.N. Population Division, while the number of residents 60 and older is predicted to explode from 16.7 percent of the population in 2020 to 31.1 percent by 2050. That is far above the global average of about 20 percent.

The imbalance is worse in wealthy coastal cities with highly educated populations, such as Shanghai. Last year, people 60 and older accounted for almost 22 percent of Shanghai’s registered residents, while the birthrate was less than one child per couple.

Xie Lingli, director of the Shanghai Municipal Population and Family Planning Commission, has said that fertile couples need to have babies to “help reduce the proportion of the aging population and alleviate a workforce shortage in the future.”

Written into the country’s constitution in 1978, China’s one-child policy is arguably the most controversial mandate introduced by the ruling Communist Party to date. Couples who violate the policy face enormous fines — up to three times their annual salary in some areas — and discrimination at work.

Chinese officials have credited the policy with helping the country avoid critical strain on its natural resources, while human rights advocates have denounced abuses in the enforcement of the policy. In rural areas, some officials have forced women pregnant with a second child to undergo abortions. In addition, many couples have had sex-selective abortions, leading to an unnaturally high male-to-female ratio.

In recent years, population officials have gradually softened their stance on the one-child policy. In 2004, they allowed for more exceptions to the rule — including urban residents, members of ethnic minorities and cases in which both husband and wife are only children — and in 2007, they toned down many of their hard-line slogans.

Qiao Xiaochun, a professor at the Institute of Population Research at Peking University, said central government officials have recently been debating even more radical changes, such as allowing couples to have two children if one partner is an only child.

In July, Shanghai became the first Chinese city to launch an aggressive campaign to encourage more births.