There are medical centers that prefer the egg sharing approach in which a woman who is already going through IVF donates half of her eggs.
The procedure of causing multiple ova to develop during a single cycle involves the use of hormones at high doses, carrying a risk of ovarian hyperstimulation syndrome, in addition to the retrieval itself, which is a surgical procedure (and any invasive procedure involves risks). For a woman who is already going through IVF, there's no additional risk, whereas a woman who is donating eggs without going through IVF herself, is exposed to some risk.
In addition, in many places, it is difficult to find egg donors, so taking eggs from women already going through IVF is an easy way to find such donors, especially since many couples who need IVF in order to conceive can benefit from the reduced cost.
I hope I've answered your question :-)
Rachel Inbar
http://www.fertilitystories.com
