On 8 June 1994, a girl was born in southern China. She was left on the street, near a police station, and not found until three days later, when she was taken to the local orphanage. The name she was given there was a reminder of her origins: LuLu, a diminutive of the word for street.

Seven months later LuLu was adopted by a single white American woman, a writer who had always loved China after spending her childhood years in Taiwan and Hong Kong.

Emily Prager's latest book, Wuhu Diary, is an account of taking LuLu back to the town where she was born, four years after the adoption. Prager had always wanted her child to feel proud of being Chinese: she sent her to a Mandarin-speaking pre-school in New York's Chinatown, where none of the parents spoke English; LuLu learnt to play the lyre; and now Prager wanted her to be a child in China, if only for an initial two-month trip. But there was something else, something other than just soaking up the atmosphere - Prager hoped someone in Wuhu might shed light on LuLu's origins.

She made plans to visit the orphanage, to look at LuLu's adoption file, to meet the nurse who looked after her in those first months, to visit the street where she was found - to form a picture, somehow, of her daughter's birth. 'If paleontologists can build a race from just a jawbone,' she writes, 'surely we can glimpse a mother and a father from an entire town.'

All Prager knew of the mother was the note she left with her baby. 'This is a girl,' it read. 'Her Western birthday is 8 June. Her lunar birthday is 29 April, night-time, at the hour of 11:30pm.'

That the child was a girl would hardly seem to need stating. And yet, the emphasis on that fact is significant. Prager's conjecture is that LuLu was a victim of China's one-child policy, still strictly enforced in 1994. She reports that more than 100,000 girls are left by the road every year; those who are not adopted are supported by the state until they are 16, and then go to work in factories. While in Wuhu, Prager watched a televised debate, in which the presenter bluntly announced that 'if we were allowed to abort unwanted children at will, there would be only males in this country and no future generations'.

When they arrived, the Prager family of two faced a number of disappointments. 'A lot of things went wrong. A lot of things.' Almost as soon as they reached Wuhu, which is six hours by train from Shanghai, they learnt that Nato had bombed the Chinese embassy in Kosovo, killing three Chinese journalists. A country that days earlier had been sympathetic to them suddenly turned furiously anti-American. There were demonstrations outside the window of their hotel room, a US academic they met had been advised to stay indoors until further notice, their translator deserted them, and when they applied for a permit to visit the orphanage, they were told 'sentiment is not with you because of the bombing'.

In any case the orphanage had been torn down. They went to the place it had moved to, without a permit, and the trip ended in tears as a guard shouted at them. Later, they discovered LuLu's files had disappeared. Prager got emotional over the street where LuLu was found, but it turned out to be the wrong street. At one point she wondered, 'Is this a Communist joke?'

Yet no matter how great the disappointments, this quest says a good deal about what it is like to be an adoptive parent, or what it is like for Prager, at least. She seems to see everything as a potential clue: when LuLu is scared of a dog's bark, Prager wonders if she had been attacked by a dog during her first three days on the street; LuLu likes boats and water - Prager wonders if her parents lived on one of the houseboats she sees in Wuhu; LuLu is calmed by gardens and music - what does this say about her ancestry?

'Some of that stuff is actually genetic,' Prager says. 'It's like picking up shards of information and putting together an archaeological project. You know, a friend of mine just had a baby, and the first thing you do is you go in, and you go: "He looks like..." It's programmed into us. And there are gaps in the information I have.'

It irks her that the doctor can't tell her how tall LuLu will be when she grows up. 'With a birth child,' she explains, 'you go to the doctor, and he looks at you, and asks how tall your mother is, and he predicts how tall your child is going to be. Well, he couldn't do that for Lulu. And I want to know. I want that piece of information. I don't know why, but I do.'

As an adoptive parent, Prager says, 'your authenticity as a parent is questioned constantly. People are forever saying you're not really the mother, and she's not really your child. Lulu's had it said to her several times in school. I think it upsets her more from the point of view that she has to deal with it at all, than that she feels I'm not her real mother. She and I don't see that any more. But others see it that way.'

There was no question of not telling LuLu she was adopted very early on. 'How could I avoid it?' Prager says. 'But some kids don't ask. LuLu's always been interested in relationships between people. When she was very young, if a couple came to see us, and then one of them came without the other, she would always ask, "Where's the other person, why aren't you with them?" Adoption is such a complicated thing for a small child to grasp, and I think Lulu was really sophisticated in the way she went about it.'

Now Prager also finds herself 'on the other side of the whole race question'. 'Someone asked me what it was like having a child from another race, and I said, well, it's like having an extremely handsome boyfriend. You're conscious of this for the first 10 days, then you stop, and you're not conscious of it until someone comes up and flirts with him. Of course there's a lot more to it than that, but that consciousness is both right with you at all times and not.'

In person, Prager is quiet, thoughtful, attractive and small. A former actress, a novelist and a veteran satirical columnist, she is now in her late forties, and, contrary to the spikiness of much of her writing, there is a soft, kittenish quality about her face. Once, when she was TV critic for New York newspaper the Village Voice, she went into the office, and the editor was shocked. 'That's what you look like?' he gasped. 'I was expecting a lady in a hat with a knife!'

Though never actively part of the women's movement, Prager always thought of herself as 'a female supremacist', and was one of the early female comedy writers of the period (she is now a 'Humour Professor', as she calls the post, at New York University). She contributed to the National Lampoon in the early Seventies, and went on to write a column for Penthouse, where, she once told the TV interviewer David Letterman, she justified her presence on the grounds that as a feminist she was 'in the missionary position over there'. She wrote pieces, now collected in a book called In the Missionary Position, with titles such as 'Full Metal Jackoff' (on Oliver North), 'How To Tell if Your Girlfriend Is Dying during Rough Sex' (aimed at Penthouse-reading teenagers) and, ironically, 'In Praise of Child Substitutes', which was published in 1981, before she knew she couldn't have children.

LuLu has been watching cartoons on television. When one show has finished, she asks if she can wear jeans instead of her dress.

'No,' her mother says 'you have to wear that dress tonight. And I'm wearing a dress too. We're both wearing dresses.' Prager turns to talk to me again.

'You didn't bring a dress,' LuLu persists.

'I did so, go and look in the closet, I brought a wonderful dress.'

LuLu doesn't believe it: 'Wanna bet?' she says. She seems, in many ways, a regular American kid.

I ask LuLu what she remembers about their trip to Wuhu, and she exclaims: 'I had free breakfast!' I ask if she wants to go back. 'No,' she says, with a little mischief in her eyes.

The word is met with a frown. 'LuLu!' Prager scolds, before telling me that if she herself had her way they would go once a year, and stay in the same hotel, where the staff have become like family.

Towards the end of Wuhu Diary, Prager asks herself why she didn't 'just raise her as a total American, play-down the birth parents, and be done with it'. In response, she reprints a letter she wrote to LuLu's birth mother. It was an exercise suggested by the adoption agency, and in it Prager promises to 'instil in her a love of China, and an identity with the Chinese people'. But the real answer to the question, the real background behind this promise, lies in Prager's own relationship with the country, 'China, guardian of my memories'. Embedded in the story about taking LuLu back to China is another story about another childhood - Prager's own.

When Prager was seven, the same age LuLu is now, her parents got divorced. Her mother, who lived in Texas, remarried and had a child, and sent Emily to live with her father in Taiwan. She lived there, and in Hong Kong, for three and a half years, and never went back to live with her mother.

'I think she thought it was a really profound opportunity for me,' Prager says now of her mother's decision. 'I was very sad to lose her, but China is a very maternal place for me - very involved with motherhood.

'I loved it,' she continues, 'it was a very comforting place - people were wonderful to me, and I wandered all over the streets by myself. In those days, Taiwan was a lot like mainland China used to be - it had open sewers, there was a lot of life in the street at all times - a lot of poverty, but a lot of life.'

On that first trip, she was 'grieving for my mother'. And on this latest trip with LuLu, she was grieving for her as well. Only months before they left for China, Prager's mother died. 'It was very strange, actually,' she says when I ask her about this. 'I didn't even realise as I was doing it that I was having the same sad feeling about my mother.' Like LuLu, Prager had lost a mother. Like her own father, she became a single parent.

In the book, memories float back to her - the smell of the Chinese house she grew up in, the chef who looked after her when she was young, playing with the children of the French ambassador, even visiting an orphanage in Hong Kong - an experience that made her 'burn with shame'. The pain of the parentless children there was, she writes, 'a guilty secret that I carried with me for half a lifetime'.

Prager tells me that one of the things people have yet to recognise about adoption is that 'when you adopt a child, you become part of that child's family, as well as their becoming part of your family. I'm honorarily Chinese by virtue of having a Chinese daughter'. It would seem that LuLu has given Prager more than just the pleasures of motherhood. She has returned to her something of a childhood she has lost, brought her back to the place Prager calls the 'nurturer of my spirit'.

Before I leave, Prager replays, with some insistence, the conversation LuLu and I have just had. 'Let's try that again,' she says. 'Do you want to go back to China?' LuLu looks a little sheepish and says: 'Yes, Mom.'

The letter LuLu wrote to Emily's birth mother

16 May 1994

Dear Madam,

The adoption agency has asked me to write to you but I don't know how. I don't know what to say to a woman whose greatest tragedy is my good fortune. That you should have your daughter forced from your arms by a government who I then must pay to envelop her in mine is the stuff of which I have fought against for my entire career. That I should end up tacitly supporting this policy is my shame, and yet my fate.

What can I promise you? That your daughter will be loved and adored? Most assuredly. That she will want for nothing? You have my word. That she will be wonderfully educated and prepared for her adulthood? To the best of my ability.

If I had my way, she would actually know you. But, they tell me, she will not have a record of who you are. But I assure you she will know of your courage and love in leaving her alive so I might raise her. Of the adversity and animosity you faced in doing so.

I shall help her to feel in her heart, as I do, the gratitude and respect for the risks you faced in giving birth in a place where giving birth is not a free act. I will also instil in her, as I'm sure you would want me to, a love of China, and an identity with the Chinese people. Don't worry. She will know where she came from, that she was born of a great and ancient tradition. Perhaps someday, she would wish to go back. The history of China is, as you know, wide and long.

As your daughter becomes my daughter, I will want for her what I have always wanted for myself, really. To be confident, secure and jolly. To be at one with life. To find people and work she loves, to take pleasure in nature and art, and to find nourishment in the spiritual and to be eternally curious.

Forgive me, Madam, for my part in the ripping-off of the women of China, and in particular, of course, you. If I did not feel that your daughter would be better off with me, I assure you I would not be doing this.

As your daughter becomes my daughter, your ancestors become my ancestors, and mine become yours. It is an interesting thing and very modern. Please understand that I respect this sacred trust, and should we meet in the afterlife, we would embrace as one family, for that's what we will soon be.

I shall think of you as the years unfold as I'm sure you will think of me. My heart is with you.