Embryos can be cultured for up to six days, until they become blastocysts. At this stage it may be easier to select the best quality embryo(s). With blastocyst transfer, embryos are cultured in the laboratory incubator to the blastocyst stage before they are transferred to the womb. At this time, one or two of the best quality blastocysts are selected and then implanted into the woman’s womb. A blastocyst must successfully attach itself to the wall of the womb for a woman to become pregnant.
The procedure for blastocyst transfer is similar to that for normal embryo transfer, but instead of being implanted into the womb after two or three days, the embryos are allowed to develop for five to six days before transfer.
The blastocyst usually forms on day 5 as fluid builds within the compacted morula. A healthy blastocyst often begins hatching from its outer shell, called the zona pellucida between day 5 to day 7 after fertilization. Within 24 hours after hatching, embryo implantation after IVF (or a “natural” pregnancy) begins as the embryo invades into the uterine lining. The blastocyst releases hCG hormone (the pregnancy test hormone) which leaks into the mother’s blood as the embryo implants.
The goal of in vitro fertilization and embryo culture is to provide high quality embryos which are capable of continued development and result in live births. However, under standard IVF culture conditions, only about 25 to 60% of human embryos progress to the blastocyst stage after 5 days of culture. The low rate of embryo development has 2 main causes:
Therefore, in the past embryos were transferred to the uterus after 2 or 3 days
Many clinics are now offering blastocyst transfer as a means of improving chances of pregnancy after single embryo transfer. This is particularly useful for younger women with a good prognosis for pregnancy from IVF. Your doctor may also suggest you try blastocyst transfer if you have produced good quality embryos in a previous IVF cycle but they failed to implant in the womb. It is not normally recommended if you produce fewer than normal healthy eggs. As with embryo transfer, due to the risks of a multiple birth, you may want to consider single blastocyst transfer. Indeed, your clinic may encourage you to have only one transferred.
Not all embryos will develop to produce blastocysts in the laboratory. Embryos can stop developing at the four-cell stage (day two) and progress no further.
The embryologist may advise your consultant that in your case it is safer to consider a day two-three embryo transfer than risk having no blastocyst to transfer on day five-six. As with normal embryo transfer, due to the risks of a multiple birth if more than one blastocyst is transferred, you may want to consider single blastocyst transfer.
One problem with this is that 2 to 3-day-old embryos are normally in the fallopian tubes, not in the uterus. The embryo gets to the uterus about 80 hours after ovulation. Embryo implantation process begins about 3 days later – after blastocyst formation and hatching out of the embryonic shell have occurred.
Therefore, if in vitro culture conditions are maximized so healthy blastocysts form at a high rate, then day 5 blastocyst embryo transfer can be done. The uterine lining on day 5 should be receptive to the arriving embryo – this a more “natural” time for the embryos to be in the uterus. It is the same timing as with a natural pregnancy.The transfer is done shortly before the time for actual invasion and implantation pregnancy rates and reduce risks for multiples.
Transferring blastocysts following IVF also provides another potential benefit – reducing possibility for multiple pregnancy. Many 2 or 3-day-old embryos do not have the capacity to become high quality blastocysts and make a viable pregnancy. However, on day two or three of culture we don’t have methods to determine which embryos will be viable long-term, and which will soon arrest their development. By culturing embryos to day 5 we will find that some of them have not become blastocysts – allowing us an opportunity to choose the most competent embryos for transfer. We can then transfer fewer embryos and still obtain high pregnancy success rates – with very little risk for having high order (triplets or higher) multiple pregnancies.
In the past, it was difficult to get high quality blastocysts with in vitro culture systems – unless “feeder” cells were utilized – called coculture. However, since 1998 more advanced culture media have been commercially available that (if used properly) can yield high blastocyst formation rates. Now blastocyst embryo transfer is a viable IVF treatment option for many couples.
Proficiency of the process and high excellent quality control in the laboratory suggests potentially no disadvantage to day 5 blastocyst transfer. However, if the culture environment is suboptimal, delayed embryo development and even embryonic arrest will occur in some cases. Therefore, if the culture system and laboratory quality control are inconsistent – good results will not be obtained with extended culture to day 5. Such programs will do better with day 3 transfers – putting back embryos earlier, before they are “stressed” excessively by the weak culture environment.