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Freezing embryos and the replacement of embryos

Why freeze embryos?

The vast majority of in vitro fertilisation cycles (including ICSI) use fertility drugs to stimulate oocyte (egg) production. This means there are often more
embryos available than can be replaced in that cycle. Providing those ‘spare’ embryos have developed to the appropriate stage within a certain time they can be preserved by freezing and replaced at a later date, without the need for further ovarian stimulation.


This increases the overall chance of pregnancy from each oocyte pick-up. Embryo freezing is also an important way to preserve embryos in cycles where fresh replacement is inadvisable, for example, where the woman is considered at risk of developing Ovarian Hyperstimulation Syndrome (OHSS).

At what stage can embryos be frozen?

Embryos can be frozen at any stage from single cell on day one to blastocyst on day five or six. Freezing at blastocyst stage has become common practice in New Zealand in recent years. 

In cycles where an embryo transfer takes place, the best quality embryo is usually selected for transfer.  The remaining embryo(s) will then be assessed and if it/they are at the correct developmental stage and the cells are of good quality, the embryo(s) may be frozen.  Even then, embryos not regarded as suitable for freezing at day two or day three can be kept in culture to see if they become blastocysts, and be frozen at that stage. 

There is an increasing tendency to freeze at the blastocyst stage of development (day 5-7 in culture). This is because more information is gained on the embryo and it needs to be stronger to reach this stage giving an increased potential for pregnancy.  Most laboratories will expect around 90-95% of all embryos frozen at the blastocyst stage to survive freezing and thawing. 

There is also a trend both domestically and internationally to move towards what is called a freeze all or freeze only cycle. This means the plan for the cycle is to freeze all the suitable embryos instead of doing a fresh embryo transfer. The embryos must still meet developmental milestone and cell quality to be  frozen. Not all embryos created will be suitable for freezing. A freeze all cycle is often suggested due to the risk of OHSS or to altered hormone profiles that mean the egg collection cycle doesn’t create the ideal uterine environment to establish a pregnancy.

How are embryos frozen?

Embryos are frozen through vitrification, where they are placed in a solution of cryoprotectant (anti-freeze) medium which allows the cells to survive freezing and thawing. The cryoprotectant helps protect the cells while they are being frozen.

Embryos are then placed on a special straw in a tiny volume of cryoprotectant, and ‘snap-cooled’ straight to -196˚C. Straws are then placed within plastic goblets, also labeled with patient identification details, then those goblets are placed in a liquid nitrogen bank for storage.

Understanding the jargon

The terminology used for people experiencing infertility, or trying-to-conceive, can be overwhelming. 

Thawing embryos for use

For women with reasonably regular cycles, frozen/thawed embryos can be successfully replaced in a natural cycle, preventing the need for any drug administration.

The woman would normally inform the clinic within a day or two of the start of her period and the clinic would arrange a plan to track the cycle to pinpoint ovulation.


Tracking is normally by blood tests. Tracking establishes the day of ovulation during that cycle and that becomes day zero. The embryos are then thawed in synchrony with the cycle. For instance, embryos frozen on day one following oocyte pick-up will be thawed on day one post ovulation of the natural cycle.


This synchronisation ensures that the developing embryo is ready to implant when the uterine lining is at its most receptive.

Women who do not cycle or have very irregular cycles can have their embryos replaced in an artificial cycle. Different clinics have different strategies for
mimicking a natural cycle with medication, but all have two essential steps. Firstly the uterine lining is stimulated to grow (usually with estrogen tablets
or patches).


Once the lining has reached a certain thickness, it is primed with progesterone to become receptive to an implanting embryo. Artificial cycles
are becoming popular because then people can plan the exact day for embryo transfer. 

Straws containing the embryos are removed from the liquid nitrogen bank and allowed to warm rapidly. 

The straw seals are then cut and the embryos recovered.

The embryos are then passed through a series of solutions which slowly draws the cryoprotectant out of the cells. It is only when the embryos have come out of the final thawing solution and been placed in culture medium that survival of most embryos can be fully assessed.

The lab will evaluate the number of cells that have survived the freezing and thawing procedure. The laboratory needs at least 50% of the cells in the
embryo to be considered survived for the embryo to be viable and suitable for embryo transfer. Between 50-70% of day 1-3 embryos and between 90-95% of blastocyst embryos are expected to be able to survive the freezing and thawing process.

If embryos have been frozen at day 1 (pronuclear), or day 2 (four-cell), they would often be cultured overnight before replacement. During that time the thawed embryo would be expected to continue cleaving. With embryos frozen on day 3 (eight-cell) or day 5 (blastocyst) resumption of development is not usually seen as embryos are usually replaced on the same day as the thaw.

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Further considerations

Immersed in liquid nitrogen at -196˚C, all biological activity ceases within stored embryos. This means that, in theory, they can be stored for hundreds of years without degrading, but in practice, embryos are rarely stored for more than a few years and the HART Act in New Zealand imposes a time limit for storage of ten years. Even so, quite a lot can happen to a couple over that time!

All clinics will want to know the wishes of the couple on what they want done with stored embryos should the couple separate or should one partner
die, and these difficult decisions have to be made before any embryos are stored. As clinics continually improve chances of success with all treatments, it is likely that many couples may be in a position where they have decided that their families are complete, but embryos remain in storage.

The options open to the couple at this stage are to remove the embryos from storage and allow them to succumb (embryos are then either discarded by the clinic or may be returned to the couple) or to donate the embryos to another infertile couple. Embryo donation is a complex process that requires ethics committee approval.

Perhaps the most important point to remember is that you, the couple, are responsible for informing the clinic at which your embryos are stored if you move or your circumstances change. It may come as a surprise to know that embryo storage banks all over the world, including New Zealand, contain embryos belonging to untraceable couples. Eventually, at the end of the legal storage period, these embryos are removed from the banks and disposed of, without the couples ever being able to be informed. If you move, let the clinic know! If you move to a long way from the clinic, or even overseas, it is possible to ship frozen embryos to another more convenient clinic.

While almost all couples choose to freeze ‘spare’ embryos, you should think about the implications before you make this choice.

Embryo replacement

Your clinic will have protocol around communication of embryo survival. If this isn’t explained to you beforehand, ask if you will be called or if you should
call the lab. Often this would be on the same day as the intended embryo replacement.


The embryo replacement procedure is the same as that used for replacing fresh IVF or ICSI embryos. Women having their embryos replaced in an artificial cycle will need to carry on progesterone support until the time of a pregnancy test (and beyond, if they do become pregnant). Pregnancy testing is around fourteen days after ‘ovulation’ for both types of cycle.

The chances of success

As with all fertility treatments, the chance of success depends on several factors, such as the woman’s age. With blastocyst transfers and vitrification, frozen embryo transfer rates are now comparable or slightly higher than fresh embryo transfer rates.

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