Desirism - A worked example?

Prompted by this idiocy, I started pondering about the issue of abortion and I thought maybe a worked example how desirism might help us make moral decisions would be greatly aid my comprehension of it.

With that in mind, would it be possible to explain how we might decide:

• Whether to abort a foetus when the life of the mother is threatened?
• At what age it might be ethical to do so if the foetus was not threatening the life of the mother.?