With many families not fitting the 'traditional model', the law is struggling to define who and what a parent is.
In 2008 Thomas Beatie hit the headlines when he became the first legally-recognised man to give birth.
Beatie had been born female and undergone gender reassignment, but had retained his female reproductive capacity. Thomas' wife Nancy was unable to have children, so when the Beaties decided they wanted to start a family, Thomas stopped taking his testosterone injections and used sperm from an anonymous donor to become pregnant.
Thomas and Nancy went on to have three children in this way. They see themselves as father and mother respectively to their children. But how would the law regard them?
If Thomas had given birth in the UK rather than in the United States, it is possible that he – rather than Nancy – would be his children's legal mother. Under the 2008 Human Fertilisation and Embryology Act (HFEA) the status of 'mother' is automatically granted to the 'woman' who gives birth to a child. However, although Thomas has given birth, he is no longer a legally-recognised woman.
This provision doesn't just have implications for transgender parents. It also means that legally there is no such thing as a 'genetic' mother. So, for example, a woman who provides her eggs in a surrogacy situation with the intent to raise the child once it has been born, has no direct legal claim on any resulting child.
These issues are of particular interest to Dr Julie McCandless, a lecturer in medical and family law at LSE.
"The HFEA is interesting because it's the only place where there is an explicit statement in statute, rather than through developments in common law, which says who a parent is and on what basis," says McCandless.
Despite the fact that the 1990 Act was reformed in 2008, its provisions concerning parenthood remain informed by very traditional and deep-rooted assumptions about family.
For example, like the 1990 law, the 2008 Act states that a child may only have two parents and explicitly only one father. This obviously affects homosexual men who have children. And it also doesn't allow for other complex situations.
An embryo mix-up case at a Leeds fertility clinic is one such example of this. There, a woman's eggs were fertilised with the wrong man's sperm and resulted in twins being born. The mistake became apparent because the woman and her husband who were seeking treatment were white and the sperm was from a black man who had visited the clinic on the same day.
So who was the father – was it the woman's husband or was it the unwitting sperm 'donor'? The Court decided that the man whose sperm was used, was the legal father of the children but that he wouldn't have parental responsibility. The woman's husband, on the other hand, had to go through the laborious process of adoption to become the legal father to his wife's children.
"Legislation could provide a discretionary provision for the judiciary if a situation doesn't fit into the two-parent family model, or if there are special circumstances," argues McCandless. "In examples such as this there could, for example, be some flexibility to award three – or more – parents with social and legal rights and responsibilities in a way that better reflected their relationships to the children."
Similarly the incest prohibition, a new provision inserted into the 2008 Act, is another example of the legislation trying to impose a particular family model. Designed to stop those with close family connections from seeking fertility treatment together, the provision prevents, for example, a mother and daughter, or a brother and sister from being treated as a child's legal parents. So while in reality we see many examples of collaborative care between family members – a teenage mother whose own parents undertake considerable parenting duties for their grandchild for example – such family members are not eligible for legal status.
Examples like these show how, by framing parenthood provisions along traditional 'biogenetic' lines and ideas about what a family 'should look like', the new legislation has not grappled with the question of what parenthood should be grounded on.
The 2008 Act does include one quite significant change which is the extension of the possibility of legal parenthood to a second woman. This means that a lesbian couple – or indeed any two women – can be considered as the child's legal parents from the moment of birth and registered as such on the child's birth certificate. And this means that such female co-mothers no longer need to adopt their 'own' children.
But what about the Beaties' case? Should a legal man be also a child's legal mother?
"The law is in part creating these complex situations by insisting that gender is a dichotomised thing, but in the future we may have to think about a gender-neutral status for legal parenthood," argues McCandless.
"More generally, the parenthood section of this legislation needs to be separated off and dealt with in a more full way across government departments. An investigation by the Law Commission, for example, could consider how it relates to other family law principles.
"Most people do not get challenged on their parental status. It is the people on the margins that get challenged and they are the ones that need the protection of the law more than others," she says.
"I don't want to overstate the law here. People construct their families without thinking about the law and things happen in spite of it. But what's the point in the law if it doesn't protect the most vulnerable?"
For full details of Dr Julie McCandless' research and publications see her profile on the LSE experts directory: Julie McCandless