In IVF or ICSI therapy, the fertilised eggs are in the pronuclear stage the morning after follicle punction. These pronuclear stage cells are further cultured. However, if several fertilised eggs are viable, some can be frozen (cryopreservation)for later use. Storage takes place in liquid nitrogen at -196 degrees Celsius and is can be maintained for many years. If the first treatment does not result in pregnancy, or if further therapy is desired after the birth of a child, the frozen cells can be thawed and cultured without having to repeat the stimulation and egg collection.

If cryopreservation of surplus pronuclear stage cells is not desired, they will be discarded. Unfortunately, cryopreservation of pronuclear stage cells and the subsequent thaw cycles are not covered by health insurance.

In principle, all female and male germ cells can be frozen unfertilised or fertilised. Sperm and testicular tissue (see testicular biopsy) can for example also be cryopreserved. A patient may choose to freeze sperm before a planned testicular operation or chemotherapy, even if there is currently no desire to have children. Likewise, donor sperm for donor inseminations is stored and shipped frozen.

Unfertilised eggs can also be retrieved and frozen for fertilisation at a later date.

Mature embryos, usually blastocysts, can also be frozen and stored for years. However, the goal is never to obtain embryos for storage.