A surrogate mother has no genetic link to the child she bears.
A couple's embryo is implanted into her womb and she acts as the oven, as it were, for nine months, before giving birth and handing the baby to the genetic parents.
And it all works out just neatly. A couple gets a baby that is biologically theirs and the 'host' or 'birth' mother isn't 'giving' up a baby that is hers; she's handing over a baby that is actually someone else's.
In the real world, it very often doesn't work out like that at all.
For a start, it almost always involves money - couples desperate for a child and desperately poor women.
And it's the commercialisation of surrogacy that has opened up the potential for its abuse.
Clearly there's a difference between a desperately poor 'career' surrogate and, say a relative or friend who isn't motivated by money but by an altruistic desire to help a childless couple.
No one really believes that a desperately poor woman has really 'chosen' to be a surrogate. Clearly it's because she has no choice but to do it to make money.
It's the money at the end of the pregnancy that's the motive and not the baby. Her womb and her body are 'commodities'.
Typically, the expectant parents have a lot more money; so it can be seen as the rich exploiting the poor.
And what about the motives of a couple who want a surrogate baby. Why can't they adopt? Is it because they only want a genetically-related baby?
And isn't that wholly narcissistic? To only want a Xerox of yourself and your partner when there are children desperately in need of loving parents who could be adopted?
The ethical, moral and medical implications of what happens when the baby isn't 'perfect' in the eyes of prospective parents are also an issue,
This was seen in the case of baby Gammy, a Down syndrome twin who was reportedly rejected by his prospective parents. It's also been reported that they wanted her to have a termination.
A host mother can also become attached to a baby during the pregnancy, physically and psychologically and may change her mind.
There have been cases where surrogates have refused to hand over the babies; the issue having to be resolved in court.
There's no doubt that surrogacy has helped couples become parents and things often conclude happily with the birth of a healthy child.
But there's also no doubt that wombs and women and babies are not things to be bought and sold. Sadly, in a world of rich people and poor people storks often become vultures.