When she was two, Yanan became one of those millions of children who sees her parents at most once a year, at Lunar New Year. If she is lucky they will be back for up to a month, although they never do that at the same time, and sometimes they are only home for a few days. Some years one of them will not return home at all; last year it was her father's turn to stay away.

The mainland is experiencing runaway economic growth of 11 per cent per year, but 'these children are paying the price', said Chen Xuemei of Save the Children. 'And that price is pretty high. Many of them feel as if they are living under someone else's roof. They really miss their parents, they miss their love. Yeye and nainai take care of them but there is a huge generation gap with the over-60s. It can become especially big at puberty, when the children don't want to talk to their grandparents about their feelings or their physical changes and their grandparents don't make an effort to understand them.'

Dubbed 'rearguard children', or lishou ertong, by the government and media, society is beginning to take notice of this giant group of semi-orphans in its midst, with a recent report by the All-China Women's Federation and UN children's fund Unicef drawing wide attention. In the report, home alone children are classified as those aged 17 or under whose one or both parents live elsewhere for work.

Awareness of the problem received a tragic boost when

12-year-old Zhang Yangyu hanged himself on February 25 at his grandparents' home in the village of Tiantai, in Anhui province , in protest at his parents' absence. Yangyu was a bright boy and deputy class monitor, and had been close to his mother, sharing a bed until she left in 2004 for Danyang city in Jiangsu province . Although neighbours and friends say Yangyu had not shown any signs of being depressed, he had expected his parents to return home in March to build a house. When they cancelled those plans he hanged himself from a rafter of a former pig pen behind the family ancestral shrine.

Why parents are choosing to leave their children behind is simple economics. 'Stay behind in the village and you can earn maybe 10,000 yuan [HK$11,155] a year,' said Yanan's uncle, who stayed to care for his parents and lived next door with his wife and 17-year-old son. 'Go and if you are lucky and work hard you can earn 20,000.'

It is spring and in Balizhuang, where the houses and paths are all made of the same tan earth, giving the village a moulded look, trees are putting out shoots, lifting the monotony of the flat, brown land.

The annual post-Spring Festival exodus to the cities was so large that of 700 inhabitants just 400 lived in the village year-round, said Yanan's grandfather Zhang Xinzhi , 68. The other 300 return for the holiday and, sometimes, for the twice-yearly harvests in June and September.

At Yanan's school, a low, two-storey, white building set in a large compound planted with flowers and trees five minutes' walk from her grandparents' home, hundreds of children dressed in bright colours shriek as they rush around during lunch break. Two of Yanan's classmates are also entirely parentless; others have one parent in the cities.

'We do okay,' said Zhang Zhiye, a serious-faced boy wearing the red scarf of the Young Pioneer organisation. Liu Yu, a tall, pale-skinned girl who is also 12 and sits in front of Yanan in the classroom, agrees. So does Yanan's grandmother, Yu Enlan , 65. 'We're all used to the way it is,' she said.

Out of earshot of her grandmother, Yanan opens up a little more. Walking down a narrow path on the way to school, clutching her dog-eared schoolbooks tightly to her chest, she said: 'I miss them when they call. And then I get angry with them for being away.'

Zheng Xinrong , chairwoman of the education department at Beijing's Normal University, said too many parents saw raising children merely as a physical challenge, and did not fully understand their children's emotional needs.

'I have met a lot of people who think raising a child is a question of having a carer, some rice porridge and some milk powder, and don't consider education, socialisation, feelings and all that.'

There are other issues, too. 'Grandparents often are poorly educated, and add on to that the fact that sometimes they are taking care of multiple grandchildren, all the educational responsibility is shifted onto the schools, but because school resources are poor, teachers may be aware of the special needs that home alone children have but aren't able to fulfil them,' said Jiang Yongping of the All-China Women's Federation legal and policy department.

In their report, the federation and Unicef found that home alone children were the second-most vulnerable group in the country to kidnapping and accidents, coming just after those children who accompanied their migrant labourer parents to the city. Often, grandparents didn't have the energy to look out for them all day long.

Save the Children's Chen Xuemei said: 'When they grow up they may be more socially isolated and lonely. Also we are concerned about bullying at school, as they are looked down on. Many are ashamed their parents are out working, as the work they do is considered a type of exploitation. This can damage their self-esteem.'

The key reason children get left behind is lack of access to education in the cities, experts agree. Yanan's uncle confirmed that. 'We can't afford the education in Shanghai. Here in the village it's cheaper.'

'It's a pretty complicated problem,' said Du Xiaoshan , deputy director of the Rural Development Institute at the Chinese Academy of Social Sciences.

'Migrants can double their income by working in the city, but it's still not enough to pay for a more expensive city education. And then they often change jobs a lot, and homes. Their situation is unstable and not suitable for bringing up children. We need to let them earn more and then their children can join them.'

Many of the children are left behind at a very young age. According to the report, nearly 20 per cent of parents left when their children were under a year, with a total of 27 per cent under five. Six-to 10-year-olds made up a further 35 per cent.

Worryingly for health professionals and educationists, 30 per cent of all one-year-olds' parents left when their child was under three months old, meaning they were not receiving breast milk.

While Save the Children had been tracking the problem since 2000, it became seriously alarmed after tainted milk powder killed dozens of infants in 2004 in Fuyang , in Anhui province , and in other parts of the country. 'That issue made us pay attention,' Ms Chen said. With their mothers gone, the group feared increasing numbers of children would be exposed to unsafe milk substitutes.

To help counter the problem, Save the Children is working with local governments to set up a network of children's activity centres where they will invite local children, educators and health care professionals to focus on home alone children's educational, psychological and hygiene needs. Children are encouraged to keep a diary to share with their parents, and in theory can seek help with homework - help grandparents may not be able to provide - or counselling.

So far, the group has about 50 centres around the country, concentrated in Anhui, Tibet and Jilin. In reality, the country would need one in each of its nearly 20,000 county towns - an almost impossible task, experts say.

Given China's fast pace of economic growth and persistent poverty in the countryside, most observers agree the situation is unlikely to improve any time soon.

'This is a major trend and we cannot stop parents going to the cities to work because that is their free choice,' Ms Chen said. 'But what we are trying to do is strengthen their parenting skills and the emotional link to their children.'

Yanan's grandmother said her son and his wife, who met in Shanghai, did not plan to return any time soon. 'She's a good girl and really understands how things are,' said her grandmother, smiling fondly at Yanan. With her grandmother watching, Yanan gives the impression of being content. 'When ma and pa call I don't give them a hard time because I know it's hard for them too and I don't want to trouble them,' she said.