Vitrification: why the speed of freezing matters

A living cell is mostly water. Cooled slowly, water forms ice crystals, and an ice crystal inside a cell tears its structure apart. That is the whole problem of freezing reproductive cells, and every freezing technology turns on that single question.

What fast freezing does

Vitrification — literally "turning to glass" — cools the cell so quickly that the water never gets the chance to crystallise and instead enters a glass-like state. Beforehand the cell is placed in protective solutions that replace part of the water inside it. The combination of the two, protective solution and extremely rapid cooling, is what separates modern freezing from the slow methods that came before.

What gets frozen

Sperm, eggs and embryos, each for a different reason. Sperm before cancer treatment or certain surgery, or when the quality of the sample is declining. Eggs when pregnancy has been postponed, or when a treatment is ahead that may affect the ovaries. Embryos when more than one good-quality embryo has formed in a cycle, or when a transfer has to be delayed.

Thawing, and afterwards

A sample is assessed again after thawing and is only used if it is suitable. That second assessment matters: freezing does not guarantee that every sample survives it, and the most honest thing that can be said about freezing is that it preserves an opportunity, not that it guarantees an outcome.

A note about time

Frozen eggs keep the age they had at the moment of freezing; years in the tank do not reduce their quality. Ovarian reserve, however, does decline over those same years. That is why a conversation about preserving fertility is really a conversation about timing.