The principle
Slow freezing formed ice crystals that tore membranes. Vitrification cools the embryo by thousands of degrees a minute in the presence of cryoprotectants: water passes into a glassy state without ever crystallising. The change is substantial — survival rates on thawing now exceed 90 %, against roughly half before.
What it changed
- Single embryo transfer became the rule. Since the others survive freezing, there is no need to transfer two to avoid wasting a cycle. Twin pregnancies — the main obstetric risk of IVF — fell accordingly.
- One collection can yield several attempts. Which is why the only useful figure is the cumulative birth rate per collection, not per transfer.
- Stimulation and transfer can be separated. Where there is a risk of hyperstimulation, or the lining is not ready, everything is frozen and transferred a cycle later in better conditions.
Children from frozen embryos
They do well. Large series find no excess of malformations; birth weight is if anything higher than after fresh transfer, offset by a slightly raised risk of hypertensive disorders of pregnancy in some endometrial preparation protocols. These differences are small and monitored.
Frozen transfer in practice
Two ways to prepare the uterus: follow the natural cycle, tracking ovulation, or replace it with oestrogen and progesterone. The natural cycle avoids drugs and appears associated with fewer hypertensive complications; the substituted cycle is easier to schedule. The transfer itself is identical to a fresh one.
How long can embryos be kept?
Technically, a very long time: births have followed storage of decades, and duration does not appear to harm results. Legally it is another matter: maximum duration, annual consent, agreement of both partners, and the fate of embryos on separation or death are all national law — and the commonest source of dispute at the end of a journey.