Unfertilized egg cells could previously be frozen, but had a very poor chance of survival after thawing and therefore only a low pregnancy rate. Since 2010, however, thanks to a new freezing technique - vitrification - it has also been possible to freeze these cells very well. If more egg cells have been fertilized as part of an in vitro fertilization (IVF treatment) or ICSI treatment than are intended for the embryo transfer, they can be frozen in the pronuclear stage (one fertilized egg cell up to a maximum of 20 hours after fertilization). so that they are available for any further embryo transfers that may be necessary or requested by you, without the need for renewed stimulation treatment. These egg cells can be stored at -196 °C for years. Only about 20% of the egg cells are damaged during freeze-preservation.