Chen Xi once saw the one-child policy as a brick wall, unyielding and inevitable. Now she considers it a nuisance.

The turning point came in November when, just as she began the fifth month of her pregnancy, Beijing announced a big change to the contentious policy, allowing couples to have two children if one parent is an only child. Chen, a 28-year-old employee at a state-owned enterprise, should qualify – her husband does have siblings, but she does not.

Yet her hopes may be dashed: although she is pregnant with her first child, she lives with her husband's 16-year-old daughter from a previous marriage, and family planning officials may consider the teenager her own.

As Chen fruitlessly searched the internet for details, her emotions turned from hope, to confusion, to anger – first at the lack of information, then at the policy as a whole. "This policy has so many downsides – it violates natural law, it makes kids spoilt and thankless," she said. "Sooner or later, they're going to have to give it up. It's really just a matter of time."

Chen is not alone. While experts doubt the relaxation will deliver a baby boom, they say it has delivered something else entirely: a paradigm shift for many Chinese people who, over three decades, have grown numb to the government's role in their reproductive affairs.

The policy's pitfalls are common knowledge: it has engendered an economically perilous demographic crunch and human rights abuses such as forced late-term abortions, abducted infants and the use of violence to collect fines.

Yet "resistance against the policy has never really been that strong", said Wang Feng, an expert on China's demographics at the University of California, Irvine. "That's why I think this top-down change – when the government says 'now the policy has outlived its use and needs to be changed' – that actually triggers a change in thinking."

Five years ago, Wang estimated, only three out of 10 Chinese people were adamant that the policy should be scrapped. "Now, with the announcement of this change, it may not be an exaggeration to see a shift to the other way around. Maybe nine out of 10 would say it's about time to get rid of this."

Last week, the wealthy coastal province of Zhejiang became the first to sign the revision into law. Hubei in central China and Guangxi in the south could follow suit by March, state media say.

Authorities say the policy has prevented 400m births since its implementation in the late 1970s. For evidence, they often point to UN estimates that the country's birthrate has dropped from 4.77 births per woman in the 1970s to 1.64 in 2011. Yet experts have called the figure into question. China's greatest fertility drop-off occurred in the decade before the policy was introduced, they say; its continuing decline mirrors that of other developing nations.

China already allows many people to have two children, such as couples who are both only children, and ethnic minorities.

Yet many of these families refrain, unwilling to bear the economic cost of a second child. Since the early 1990s, "there's been no tightening up of the policy, but the fertility rate continues to decline", said Zuo Xuejin, an executive vice-president at the Shanghai Academy of Social Sciences. "So the basic driver for declined fertility is socioeconomic change."

If China's demographic trends hold, the country will probably scrap the policy by 2020, according to Zuo. "By 2025, the government will be encouraging people to have more children."

Yet the policy will almost certainly continue to have a hold on people for years, experts say, for reasons that have little to do with demographics. For leading officials, backtracking on a three-decade-old policy would entail an intolerable loss of face. The country's sprawling family planning bureaucracy, which levies more than £1bn annually in fines, is too deeply entrenched to suddenly dismantle.

"At the same time as China announces this policy change, it says the government will continue to put population control as its main mission," said Cai Yong, a sociology professor at the University of North Carolina, Chapel Hill. "You see a contradiction right there."

Chen is still waiting for details on her eligibility, and probably won't find answers until Beijing formally legislates. She badly wants a second child, unlike many of her colleagues. "If my child doesn't have any siblings, she'll grow up very lonely," she said. If she finds out it is illegal, she will book a flight to the US and hopefully give birth there, granting her child foreign citizenship.

"Before, most people would go to Canada, but some policies changed, so people don't go there any more," she said. "Going to America is really popular. If that's what it takes, that's what I'll do."