Sometimes there are only few days available before chemotherapy or other medical treatment that threatens future fertility in females. Sometimes hormonal exposure that comes with the use of fertility drugs may not be desirable. In these instances an egg retrieval can be attempted without any ovarian stimulation or sometimes only after a brief ovarian stimulation. Because of the lack or brevity of the stimulation, thus collected eggs are typically immature and need to be matured in the petri dish. Using a specialized culture media these immature eggs can be matured over the next 24-48 hours. After that depending on whether egg or embryo freezing is desired, they can either be frozen as eggs or as embryos after being fertilized my ICSI.
Pregnancy rates after IVF with in vitro matured eggs is lower than when already mature eggs are used. Hence this approach is used when there is insufficient time or when there is a concern about being exposed to high levels of estrogen such as in breast cancer.