Embryo freezing: frozen is not "second best".

Embryo freezing (cryopreservation) cools IVF embryos to minus 196°C in liquid nitrogen, where all biological activity stops, so they can be used later.

The danger in freezing is ice: crystals tear a cell apart. Older slow freezing cooled embryos gradually and lost many of them. Vitrification uses a protective solution and cools the embryo almost instantly, so the water turns to a glass-like state instead of ice. It is now the standard method worldwide.

Most clinics freeze embryos at the blastocyst stage, on day 5 or 6, because an embryo that reaches this stage has passed a real test of development.

Some are frozen earlier, on day 2 or 3, when numbers are small. Embryos that have stopped dividing are not frozen, because warming them would not give anything to transfer. Your embryologist tells you how many were frozen and at what grade. Grading describes how an embryo looks, not what it will become.

No: for most women, freezing all embryos and transferring later gives broadly the same chance of a baby as a fresh transfer (Cochrane review, 2021).

Freezing lets the womb recover from stimulation injections before transfer. It is the safer choice when the risk of ovarian hyperstimulation syndrome (OHSS) is high, as in many women with PCOS, and it is required when embryos are tested (PGT-A). A fresh transfer skips the freeze and warm step and is quicker. Any clinic that uses one approach for every patient should explain why.

Cryopreservation in fertility began with frozen sperm in the 1950s, reached embryos in the 1980s and became reliable for eggs only when vitrification arrived in the 2000s.

In 1949 scientists found that glycerol protected sperm from freezing damage. The first births from frozen human sperm were reported in 1953. The first baby from a frozen embryo was born in Melbourne, Australia, in 1984, and the first from a frozen egg, also in Australia (Adelaide), in 1986. Slow freezing still lost many cells, until vitrification became standard in the 2000s. Today embryo freezing is a routine part of IVF, not an experiment.

Under the ART (Regulation) Act 2021, embryos can be stored for up to 10 years, and only with the written consent of both partners.

The Act (section 22) says a clinic may not freeze embryos without written instructions from both of you on what happens if either partner dies or can no longer decide. At the end of storage (section 28), embryos are allowed to perish or, with your consent, donated to a registered research organisation. Either partner can withdraw consent before an embryo is transferred. Keep the clinic updated with your address and wishes, so a decision is never made by default.

Large follow-up studies of children born after frozen embryo transfer have not shown more birth defects than after fresh transfer.

Two differences are reported. Babies after frozen transfer tend to be slightly heavier at birth, and some studies find a higher risk of high blood pressure in pregnancy, mainly in cycles prepared with hormones. Neither changes the overall picture, and both are still being studied. We discuss them with you when planning your transfer.