As with any relationship breakdown, issues to be dealt with include division of the finances, any civil partnership dissolution and with whom any children will live (as well as contact with the non-resident parent). But these types of divorce cases have a more complex dynamic, with difficult legal and social questions arising from the mismatched biological (and often legal) status of the two female parents.
To date there has been very little judicial guidance as to how much weight the family court will place on the importance of being a birth mother in divorce proceedings, and whether in such cases the birth history and biological link should be considered more important than the relationship between the non-birth mother and the child.
Of course every case is unique, but the two main cases so far where the court has considered and explored these issues in principle make for very interesting reading.
The first case was that of Re G  UKHL 43 (1) which involved a difficult dispute about where the children conceived to a lesbian couple through artificial insemination should live following their separation. The High Court and Court of Appeal ruled that the non-birth mother should have primary care of the two children (mainly because the birth mother had behaved badly and removed the children to Cornwall deliberately to obstruct her former partner's relationship with the children).
However, in a landmark judgment the House of Lords ruled that the lower courts had not given sufficient weight to the fact that the birth mother was the biological mother of the children and ordered that the children should continue to live with her. The House of Lords expressly stated that the lower courts had placed too much weight on the behaviour of the birth mother and not enough on the biological basis of her relationship. This was a 'significant consideration which was of importance'. Being the birth mother is, it seems, significant.
The more recent case of T v B  EWHC 1444 (Fam) (2) involved a lesbian couple who were not civil partners but had lived together for many years and had undergone fertility treatment to conceive a child together. Once the child was born they both undertook the role of parents. Although the law at the time did not recognise the non-birth mother as a legal parent, she sought - and was given by the court - parental responsibility, which meant she had full legal authority to take decisions as a parent and to be involved in her child's care.
Following separation the birth mother applied to the courts for financial provision from the non-birth mother. The court ruled that as the non-birth mother was not a legal parent she had no financial obligation despite the fact that she had to all intents and purposes been a 'parent' to them from the very start. The court was somewhat constrained by the wording of the law (and its frustration was evident) but it was clear in this case that whether you were a birth mother or not was deeply significant.
When the court are considering cases involving disputes about care arrangements for children, the court has a range of factors it has to take into consideration. These include: the child's age, sex and background; their physical, emotional, educational needs; the effect of any change in circumstances; their ascertainable wishes and feelings; any harm the child has suffered or is at risk of suffering and how capable each parent is in meeting the child's needs.
The welfare of the child will be the court's paramount consideration and any decision made by the court will be based on what the court considers to be in the child's best interests. In practice this gives a lot of flexibility, although it is clear that the court is inclined to place weight on the importance of the biological link with the birth mother. In relation to child maintenance questions, this bias is more institutional, with clear legal rules which make only legal parents (and their spouses) financially responsible.
On 6 April 2009 the law in the UK changed to allow two mothers to be named on the birth certificate, recognising them both as the legal parents and giving them both financial responsibility for their children. It is notable that both of the birth mother vs non-birth mother cases have involved children born before this legal change.
Whether or not the new law will give greater weight to the non-birth mother's position waits to be seen (although this will certainly be the case in relation to financial questions). Things are likely to be muddied further by the increasing blurring of the lines between birth and biological parenthood for lesbian couples. We are certainly seeing more egg swapping cases, where an egg has been taken from the non birth mother, fertilised and then transferred to the birth mother. Where parents in these situations separate, will the birth mother or the biological mother be the one with the upper hand?
Same-sex divorces are undoubtedly legally complex where children are involved. In a dispute over a child within an alternative family structure, an argument often run is the importance of the biological link, and the genetic identity of the child. With changes to the law and even more complex family structures emerging, it will be interesting to see how the court responds.