Cryopreservation of oocytes at the pronuclear stage
On the day after the oocyte retrieval during IVF or ICSI, we can see whether a sperm has penetrated an egg cell and whether both cells have become active and taken the first steps towards fertilisation successfully.
In this case, on microscopic examination, we would see two pronuclei in the oocytes - one with the father's and one with the mother's genetic information (= pronuclear diagnosis). The actual fertilisation, i.e. the fusion of the genomes from both partners, will not have taken place at this stage.
Should more oocytes reach the pronuclear stage than the patient wants to receive, we can freeze the spare pronuclear cells in a gentle, computer-controlled process and store them in liquid nitrogen at -196° C.
These cryopreserved pronuclear cells can then be transferred into the womb in a later cycle without the patient having to undergo stimulation and oocyte retrieval again. In this way, the chances of achieving a pregnancy per oocyte retrieval are increased. Statistically, 2/3 of the pronuclear cells survive cryopreservation well, and the chances of achieving pregnancy in a "thawing cycle" are around 25%.
Cryopreservation has been successfully performed globally for many years. No particular risks have been observed which would argue against the procedure. In particular, there has been no increased risk of abnormality in the resulting children.