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 five to six days. An embryo that has developed for five to six 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.
View more information on Blastocyst transfer on the HFEA website. Also see the Multiple Births Foundation's Zygosity page.
Blastocyst transfer can be highly successful for some groups of women, but it isn’t right for everyone and does have some drawbacks:
A number of recent studies showed that, although overall pregnancy rates were similar when a single blastocyst was transferred to the womb compared with double blastocyst transfer, up to 62% of pregnancies after multiple blastocyst transfer were twins.
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 two to three day embryos.
It is thought that this is because only better quality embryos will survive for five to six 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 two to five times more likely to become pregnant with identical twins after receiving fertility treatment than after natural conception.
"Where more than one good quality blastocyst is available for transfer
on day five or six of culture, the case for single blastocyst transfer is overwhelming."
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 recent study 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 two to three day (cleavage stage) embryo transfer and five to six 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.
"…new research has shown that the single embryo transfer of a good quality blastocyst during IVF treatment can not only increase a woman’s chances of falling pregnant, but it also has the desired effect of decreasing the risk of a multiple birth."
Professor Philip Steer, editor-in-chief of BJOG: An International Journal of Obstetrics and Gynaecology.