The chances to achieve pregnancy after retrieving >15 oocytes are not good, probably because of bad effect on the endometrium (uterine inner lining). Therefore, in most such cases the investigators prefer to freeze all embryos, and transfer in a thaw cycle. With the introduction of oocyte vitrification (very fast cooling) technology, a new option emerged: to freeze un-fertilized eggs, to be fertilized and used later in a thaw cycle.