Among many other regulations, the convention stipulates that international adoptions must be overseen by a central - read government - authority. The introduction of the convention was a bid in part to cut out unscrupulous private operators and make the issue more transparent. Both China and Hong Kong have central authorities. On the mainland, it is the China Centre of Adoption Affairs (CCAA), here the Social Welfare Department.

That regulation has led to the United States recently cutting off adoptions - at least temporarily - from Guatemala, a key area for adoptive parents from the US. Guatemala lacks a central authority.

Elizabeth Bartholet, faculty director of the Child Advocacy Programme at Harvard Law School, who has written extensively on the issue of international adoption and law for the past 20 years, feels the convention is all right in itself but that it is creating barriers to the adoption of needy street children because of its interpretation by international organisations and the US government.

'I'm mostly concerned about how it is being falsely portrayed,' she says, adding that the US usually takes 4,000 to 5,000 children from Guatemala a year. 'Last year, 4,135 children came to the US. Guatemala was the second largest country after China, where 6,493 children went to the US.'

The reason potential parents focused on Guatemala, she says, was because they released children at just six months to a year - before they became institutionalised. 'By the time they are three to four years old they are significantly destroyed' by being in an institution. She says there are good private agencies, and all the adoption work would be undone if the state took over. Guatemala is among several Latin American countries, she feels, that do not have the infrastructure for a central authority. Ensuring adoptions are legal and preventing trafficking is down to stringent local law enforcement in any country, she says.

International organisations, including Unicef, the UN's children's rights wing, are also keen that a child should stay in its own country because of issues of heritage and contact with its cultural background. While Professor Bartholet does not argue with this in theory - she has two children adopted from Peru who she has taken back for visits when they were teenagers - she feels the issue of heritage is overdone.

'Let's not over-emphasise the heritage issue,' she says, 'if that means that they stay on the streets of Guatemala or in a Guatemalan orphanage.' A loving home is far more important, she argues.

While for the adoptive family, an international adoption can be very positive, it is still very difficult for parents to track exactly where their child came from. Another issue is that once the child is moved to a foreign country, how important is it for the adoptive family - often racially and ethnically different from their new offspring - to provide a cultural and ethnic context for their children?

American Brian Stuy has three adopted daughters from China. He runs Research-China, a website which helps adoptive families track back to when their children were first found in China, sometimes helping them to find where their children were left at institutions, or the first 'finding photo' issued in local newspapers.

'I started the research in 2001, doing it basically for my daughters,' he says, so that when they were older and starting to ask questions about their origins he could hopefully provide answers. This grew to providing a service for other families. His view is that providing his daughters and others with a sense of who they are could be important to them, so it is vital to obtain as much of that initial data as possible.

Briton Chris Atkins, founder member of the London-based Transnational and Transracial Adoption Group, says it is important that adoptees have a connection to where they came from. Ms Atkins was born in Hong Kong, probably the daughter of mainland migrants, in the early 1960s.

She was adopted and taken to Britain. While her adoptive parents have been incredibly loving, she says, it was difficult growing up being noticeably different; she encountered racism at school, and Hong Kong was just two words to her.

She did not even know where it was on a map. It is only in two recent trips here that she has been able to track where she came from.

Tracking exactly where your baby came from in mainland China is exceptionally difficult, says Mr Stuy.

Professor Bartholet argues that parents can consult a plethora of adoption organisations in the US to seek out the more ethical agencies and orphanages. They can quiz the orphanage management on trips to China, she says. But even then, says Mr Stuy, parents often have to trust the word of the orphanages.

He classes some of the orphanage administrators among his friends on the many trips he has made back to China for his research with his Chinese wife, but there are of course others that are less ethical. 'Unless the family goes to the city and directly interviews the people who found the child, it is very difficult for an adoptive family to check that the information is accurate.'

The CCAA, he claims, has become less free with information. 'When we adopted our oldest daughter, [now aged 10], we visited the finding location and the orphanage. Now parents meet the kids in the provincial capitals.'

For many parents a question mark over the origins of their child will always remain.

In May, China introduced a swathe of new regulations implementing more stringent health conditions on prospective parents - no obese parents, no one who is sight-impaired, and no one with facial deformities were among some of the stipulations. 'China has also locked out single parents,' says Professor Bartholet.

This was met by negativity in the US - a key adopter of Chinese children. In the view of many American prospective parents, agencies and other organisations, says Mr Stuy, there are still thousands of abandoned girls languishing in China's orphanages.

Not so, says China, and Mr Stuy tends to believe it. Also more families within China want to adopt. With fewer children available, some unethical orphanages have made the price unaffordable for local parents, who have to wait two to five years to adopt a healthy baby.

'Every month you can read about another baby trafficking scandal,' he says, because the demand is not being met domestically.

Mr Stuy's advice to foreign parents? Adopt a special needs child, because that's the direction he feels China will be moving in within two years. 'The ratio of special needs to healthy children is changing.'

David Youtz agrees. He is the father of four girls adopted from China - one 12-year-old and three-year-old triplets - as well as being the chief executive of Mother's Choice, a non-profit organisation that helps pregnant teenaged women in Hong Kong. The reasons for the reduced number of healthy babies on the mainland, he says, is unclear and could be for a number of reasons.

'Selective termination, widespread use of ultrasound' are two reasons but not the complete picture, Mr Youtz says.

'But there are many children with special needs. Hopefully one of the outcomes [of the tightened regulations] will be that more special needs children are placed [abroad] and that there is also some domestic adoption of children with special needs.'