If the woman does not live in a stable partnership, the only way to fertilise the oocytes at the time of retrieval would be to use donated sperm. However, such a procedure, which is called heterologous IVF, is unacceptable to most patients and illegal for unmarried patients in . Progress in the preservation of unfertilised oocytes has opened up new opportunities in this field.

The first report on a pregnancy after fertilisation of cryopreserved oocytes has been published in the 1980s. However, it is only through improved lab techniques, in particular optimising the highly sensitive freezing and thawing processes, that pregnancy rates can be achieved which justify the use of this method for special indications. As with fertilised cells, mainly slow programmed freezing protocols have been used also for unfertilised cells. 
 

Studies on the structural integrity of cryopreserved oocytes (Gook et al. Hum Reprod 1994, 9:684-691; Baka et al. Hum Reprod 1995, 10:1816-1820) as well as the birth of children without major deformities (Yang et al. 2002) suggest that the cryopreservation of unfertilised oocytes prior to cytotoxic therapy is a safe method. 
 

However, the success rate of the cryopreservation of unfertilised oocytes seems to depend on the freezing protocols used. According to recent findings based on a metaanalysis, the survival and implantation rate seems to be lower than that of fertilised oocytes when slow freezing protocols are used (Oktay et al., 2006, 86: 70-80). 
 

According to this metaanalysis, the so-called vitrification promises higher success rates, a finding confirmed by recent studies by other groups. In vitrification, the cells are put into small volumes of high-molecular weight cryoprotective solution in special containers (e.g. straws, see photo). They are then immersed directly in liquid nitrogen, so that they are frozen ultrafast and without crystallisation. The resulting crystal-free solutions are crystal-clear (vitrous). By vitrifying unfertilised oocytes, the survival and pregnancy rates achieved seem to be similar to that of fertilised oocytes.

Irrespective of the freezing technique used, the success rate depends heavily on the expertise of the centre. Therefore, unfertilised oocytes should only be cryopreserved if the centre can prove it is experienced and can show own data.