When Wu Rui's 12-year-old daughter died she lost not just the only child she would ever have but also her source of security and support in old age.

Today the 55-year-old takes care of herself and her own elderly parents on a paltry pension in a ramshackle two-room home, living in fear of medical emergencies she has no way to pay for.

China's one-child policy normally leaves four grandparents and two parents relying on a single caretaker for old age -- and bereaved families with none.

An estimated one million families nationwide have lost their sole descendant since the measure took effect in 1980, and another four to seven million are expected to do so in the next 20 to 30 years.

Many, like Wu, will have no one to help them through the frailties or medical costs of old age.

"If I have a big illness then I probably won't have enough," she says quietly. "For sure there will be difficulties."

Wu divorced in 1994 and lost her daughter Zhang Weina one year later after a long struggle with epilepsy.

She now spends much of her time at home, knitting sweaters and preparing food in a cramped kitchen -- which doubles as her 76-year-old mother's bedroom.

Her 80-year-old father, his hearing failing, sits one bed over in the narrow room they share. Two light bulbs dangle from a rope and cracked paint covers the walls.

Aside from ill health, Wu's biggest fear is that their dingy but inexpensive home will soon be demolished, as many old Beijing residences have been.

The other half of their centrally located neighbourhood has already been replaced by modern towers, and if their alleyway is next they may be moved to an apartment that costs more than her monthly pension of 2,000 yuan ($310).

-- One-child limit creating old-age bubble --

Since 2001 national law has required local governments to provide "the necessary help" to families who lose their only child, but does not define what that entails.

Regulations vary by area, with Sichuan province allowing families to apply to have another child while Shanghai stipulates a one-time payment of an unspecified amount.

Some local governments provide small stipends, according to state news agency Xinhua, while a Beijing official told local media the capital offers 200 yuan a month and "spiritual" support in the form of visits from young people.

"The rule has always been there but I don't think it's very meaningful," says Yi Fuxian, a US-based academic and author of "Big Country in an Empty Nest", which criticises China's family-planning policy.

Some 4.63 percent of China's 218 million-plus single-child families are expected to lose their son or daughter by the time they reach the age of 25, he says, citing official statistics.

That would mean more than 10 million couples outliving their only child in the next two to three decades, minus a fraction who give birth again.

Yi and other demographers argue that China must not only provide for these families but also abolish the one-child limit immediately.

Its defenders say it has helped prevent over-population and lift vast numbers of Chinese out of poverty.

But it is also creating instead an old-age bubble -- by 2050 30 percent of Chinese will be 60 or over, the UN estimates, versus 20 percent worldwide and 10 percent in China in 2000.

Without more young people, China will not have enough grandchildren to provide for their elders or workers to pay into a social security system the government is trying to build.

-- "Endless pain" --

The country can now absorb a higher birth rate without risking over-population, say Yi and others.

But the head of the State Population and Family Planning Commission Li Bin told Xinhua last year that China intended to "maintain and improve" existing measures, while understanding the need to address its ageing population.

For now certain families receive exemptions from the one-child rule, including some farmers who give birth to a girl, couples who belong to ethnic minorities and parents who are both only children themselves.

The authorities increasingly recognise the problems the one-child policy created now that its first generation of parents is entering old age, says Gu Baochang, a professor at Beijing's Renmin University.

But they should have acted years ago as demographic dangers will only swell with time, he warns. "The later they do this, the greater the pain, the bigger the costs, and the greater the number of families who lose their only child."

Families like Wu's face not only uncertain futures but also an unshakeable sense of loss in a culture that emphasises family, Gu points out.

One bereaved mother shares her grief on an online forum for parents like her: "All beauty has been pulled away, the darkness of the clouds and night conceal my endless pain."

Another parent wrote on the forum: "We responded to the call and only had one child. And where is the care and concern for us? There is none. Cancer, heart and brain disease, depression and other serious ailments keep coming knocking.

"There is no institution facing up to our existence, let alone any department that sympathises with our sorrow.

"We have fallen into lonely, bitter, tragic circumstances with no one to rely on."