Embryo transfer: 5 - 6 days

During an in vitro fertilisation (IVF) treatment cycle, eggs are removed from the ovaries and mixed with sperm in a dish in the laboratory so that fertilisation can take place. The embryos are then left to develop in the dish.

Depending on how many embryos are produced and their quality, embryos can be left in the laboratory for 5–6 days. An embryo that has developed for 5–6 days after fertilisation is known as a blastocyst.

With blastocyst transfer, embryos are allowed to develop to the blastocyst stage before they are transferred to the womb.

What are the pros and cons of blastocyst transfer?

Blastocyst transfer can be highly successful for some groups of women, but it isn’t right for everyone and does have some drawbacks:

• blastocyst transfer can result in a higher likelihood of becoming pregnant when compared with 2–3 day embryo transfer in certain groups of women.
• however, if you opt for blastocyst transfer, you may not get any embryos that develop to the blastocyst stage.
• there may also be fewer embryos to freeze.
• if your embryos do develop to the blastocyst stage and multiple blastocyst transfer is used, you are at greater risk of producing twins.
• blastocyst transfer may not be suitable for all women and not all UK centres are able to provide it. Different clinics have different policies and working arrangements. Ask your clinic whether they offer this service and how they decide who is suitable for it.

How does blastocyst transfer affect the risk of multiple birth?

Blastocyst transfer appears to increase the chance of a live birth, particularly for mothers under the age of 35. Unfortunately, when two blastocysts are transferred, it also greatly increases the risk of multiple births – some studies suggest it increases the risk even more than transferring 2–3 day embryos.

It is thought that this is because only better quality embryos will survive for 5–6 days before transfer, so they are even more likely to implant themselves in the womb. A study in 2005 found that after double blastocyst transfer the twin pregnancy rate was 62%, compared with only 3.2% after a single blastocyst transfer.

A recent study showed that you are also 2 to 5 times more likely to become pregnant with identical twins after receiving fertility treatment than after natural conception.

What are my options?

If you and/or your clinician considers that that you are at risk of multiple pregnancy, you should discuss transferring only one embryo (single embryo transfer, known as SET).

A study in 2005 showed that where younger women received only a single blastocyst, their chances of a live birth remained almost identical (65.3%) compared with those after double blastocyst transfer (64.2%), when one fresh and one frozen cycle were included. However, there were only 7% multiple pregnancies in the single blastocyst transfer group, compared with 34% in the group who chose double blastocyst transfer.

If you are young and have a good chance of becoming pregnant with blastocyst transfer, single blastocyst transfer gives a similar chance of a live birth to multiple blastocyst transfer, but with a greatly reduced risk of a multiple birth.

A study in 2007, comparing 2–3 day (cleavage stage) embryo transfer and 5–6 day embryo (blastocyst) transfer, showed that single blastocyst transfer, when used by the most suitable patients, could actually improve the overall pregnancy rate while almost halving the rate of multiple pregnancies.

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