What is a blastocyst?
Blastocysts are embryos that have been cultured for five or six days after insemination, before they are implanted onto the woman’s womb. At this stage it may be easier to select the best quality embryo/embryos. The embryo has divided many more times into many more cells, it has a very thin outer shell, thus potentially increasing the chances of implantation into the unterine cavity.
While the majority of fertilised eggs in vitro will develop into a three-day old embryo, only about 40% of these embryos will develop into a blastocyst. It is believed that those embryos that failed to develop to the blastocyst stage would not have established a pregnancy. When there are large numbers of good quality embryos available at the blastocyst stage, these can be frozen. Blastocysts are considered to be a group of embryos with a higher chance of pregnancy per embryo transferred.
A blastocyst must successfully attach itself to the wall of the womb for a woman to become pregnant.
How does blastocyst transfer work?
The procedure for blastocyst transfer is similar to that for embryo transfer. Instead of being transferred into the womb after two or three days though, the embryos are allowed to develop for five to six days before transfer.
Some researchers believe that the conditions in the womb may be more optimal for a blastocyst than a day 2/3 embryo, because there are slightly differing conditions in the fallopian tube and the womb.
The advantage of transferring blastocysts, is the high live birth rate per embryo transferred.
When is blastocyst transfer the best choice?
Blastocyst transfer is used as a method which potentially improves the chances of pregnancy. This is particularly useful for younger women with a good prognosis for pregnancy from IVF.
It is not usually recommended if the woman produces fewer than normal healthy eggs.
What are the risks of blastocyst transfer?
Not all embryos develop to produce blastocysts in the laboratory. Embryos can stop developing at the four-cell stage (day two) and progress no further, though it is believed that in the uterus those embryos might have developed to the blastocyst stage.
The embryologist may suggest it is safer to consider a day two-three embryo transfer than risk having no blastocyst to transfer on day five-six.
Due to the risks of a multiple birth, a single blastocyst may be considered for transfer.