China could scrap its one-child policy, a senior family planning official said today, acknowledging concerns about its effects in creating an ageing society and gender gap.

The controversial rules, which restrict most urban families to a single child and rural households to two, were introduced in the 1970s in a bid to bring the country's vast population – the world's largest – from soaring out of control and outstripping limited resources.

But today the vice-minister of the National Population and Family Planning Commission said officials were carrying out detailed examination of the environmental, social and other implications of changing the law.

Asked if they were planning to axe the one-child policy, Zhao Baige told reporters in Beijing that there was a "very serious process" of study.

"I cannot answer at what time or how [we will decide], but this has really become a big issue among decision makers," she said.

"We want to have a transition from control to a slowdown [relaxation], incrementally. The attitude is to do the studies, to consider it responsibly."

Although the population has yet to peak – it is expected to rise from 1.3 billion now to 1.5 billion in 2033 - the birth rate has dropped below the replacement rate of 2.1.

Rising prosperity in recent years has also helped to change attitudes. Zhao said 60% of young women now say they want a maximum of two children.

While there is no prospect of controls being thrown out overnight, changes could be rolled out region-by-region, or introduced for particular kinds of households.

Concessions already exist allowing people in their second marriage to have another baby if their spouse has none, and permitting couples without any siblings to have two children.

But officials are nervous of announcing potential changes in the rules lest people pre-empt them. Discussions about relaxations of the law in 1983 are believed to have led to the birth of an extra 30 million babies that year.

Zhao also acknowledged the problems posed by the longstanding cultural preference for boys and warned that in future the use of ultrasound to predict the sex of a child – and terminate female fetuses – could become "a big issue" for China.

It already has 118 male births for every 100 female; way above the global "normal" ratio of between 103 and 107 boys for every 100 girls.

The government is rolling out a scheme to encourage families to value girls by introducing special social and economic benefits for them.

It is developing an increasingly sophisticated set of policies around population control, focusing not just on the total number of citizens but also issues such as age distribution. It is also attempting to address the underlying causes of excess births and the preference for males, and to promote its policies more effectively.

"In the 70s it was always the same language – 'One child is best'. Now it is about giving information on contraception," said Zhao.

The enforcement system is far less punitive than in the 80s and early 90s, but families that exceed the official limits face fines or "compensation fees". These can be punitive for poorer families – which can face the confiscation of property if they fail to pay - but almost insignificant for the wealthy.

That has spawned resentment that a good income can even affect a household's ability to have children.

The commission also said that, in a case that became an international cause célèbre, two officials had been detained for three to six months, and one official sacked, after women in Shandong province were forced to have abortions and sterilisations. According to some reports, up to 7,000 women were affected.

However, Chen Guangcheng, the blind activist who tried to launch legal actions on behalf of the victims, is still imprisoned.