Once here, it must ‘implant’ in the wall of the womb and grow before we can say that IVF has resulted in a successful pregnancy. The timing of embryo transfer can vary, depending on whether the embryos being transferred are ‘fresh’ or ‘frozen’. Fresh embryo transfer refers to embryos that are transferred to the uterus 3–5 days after a woman’s eggs have been collected and fertilised by sperm. Frozen embryo transfer refers to embryos that have first been frozen before being thawed at a later date for transfer into the womb.

In the early days of IVF, fresh embryo transfer was the favoured approach. However, the techniques used for freezing and thawing of embryos have since improved to a point where more than 90% of embryos will survive the process. Consequently, a ‘freeze-all’ strategy has become more common, whereby all embryos are frozen following successful growth. They generally remain frozen for at least a month before the best embryos are thawed and transferred into the womb.

Understandably, people undergoing IVF are often eager to get pregnant as quickly as possible – and may assume that fresh embryo transfer is both faster and more effective. But is one approach better than the other?

To appreciate the pros and cons of fresh versus frozen embryo transfer, you first need to understand the so-called ‘window of uterine receptivity’.

The window of uterine receptivity

The success of embryo transfer depends on a number of factors, one of which is uterine receptivity – that is, how ready the uterus is to ‘receive’ the embryo. Outside this window of receptivity, the embryo may fail to implant in the wall of the uterus.

To receive the embryo successfully, the uterus must be ‘primed’ by the hormones oestrogen and progesterone. Under natural conditions, the priming of the uterus is perfectly timed with a woman’s monthly cycle, such that if an egg is released from the ovary and fertilised by sperm, the uterus is ready to receive the embryo. In the IVF setting, however, this timing may be less than perfect.

During IVF, the ovaries are stimulated via self-injectable medication so that the highest possible number of eggs can be collected. By artificially driving the release of so many eggs, the levels of oestrogen and progesterone skyrocket – they can rise to 10 times higher than normal peak levels. This may cause the uterus to prematurely prepare itself for embryo implantation, bringing forward the time frame in which the uterus is receptive. The problem with this is that by the time a fresh embryo is grown and ready for transfer, the window of uterine receptivity may have passed.

Frozen embryo transfer overcomes this problem by delaying the transfer process. This gives the hormone levels time to return to normal and the embryo is then transferred at a later date, when the uterus is receptive again.

When is frozen transfer best?

There are some situations where frozen embryo transfer may be considered the best option, including:

• High levels of progesterone: When progesterone levels are high at the time of egg retrieval, there is a higher chance that the window of uterine receptivity will shift forward. In these cases, it is generally better to freeze the embryos and transfer them later when the uterus is receptive again.
• Polycystic ovary syndrome (PCOS): Studies have found that women with PCOS tend to have better results from frozen versus fresh transfer. Frozen transfer reduces the risk of ovarian hyperstimulation syndrome (a complication of egg retrieval), and is also associated with a higher chance of ongoing pregnancy.
• Embryos that require genetic testing: The genetic testing of embryos takes time. Consequently, by the time a tested embryo is ready for transfer, the window of uterine receptivity is likely to be over. In this scenario, frozen embryo transfer is usually more appropriate.

What are the arguments for fresh transfer?

On the other side of the coin, fresh transfer avoids the need for the freeze-thaw process. While current technology enables a greater than 90% survival rate for frozen embryos, this level of risk may not be acceptable for some patients – especially if they already have a low number of embryos. Fresh transfer potentially also results in a shorter time to pregnancy.

Which is the best option for you?

When choosing between fresh and frozen embryo transfer, there is no one-size-fits-all approach. As with all aspects of fertility care, the decisions need to be personalised to your individual circumstances. Our fertility specialists will assess the specifics of your situation and tailor their advice accordingly.

If you would like professional advice about the next steps to take on your fertility journey, you can make an appointment with one of our fertility specialists by calling Newlife IVF on (03) 8080 8933. Alternatively, you can book online via our appointments page.