In most IVF Units embryos are normally transferred 2 to 3 days after the egg collection. However in natural cycles embryos would not normally reach the uterus until day 5, this is called the blastocyst stage of development.

Historically embryos have been cultured in the laboratory for 2 to 3 days before embryo transfer, as it was thought to be the most appropriate stage to replace the embryos back into the uterus. It also allows us to offer embryo freezing at the most successful stages.

Recently, however, new types of media have become available that enable us to keep the embryos in culture in the laboratory for longer and therefore allows us to monitor the development of the embryos over this extended time period. Around 50% of human embryos stop developing before the blastocyst stage. The advantage of following the development further is that we can select embryos that continue to develop to the correct stage. This may mean that these embryos may have greater potential than the others but it does not mean that a pregnancy will definitely occur as other factors are also important such as the endometrial lining of the uterus, age of the patient and cycle history. Pregnancy results from large numbers of blastocyst transfers are not available yet, however they are encouraging.

If blastocyst transfer has been recommended taking into consideration a number of factors (such as age and number of previous attempts), then there will be further criteria that need to be met during the cycle. One of the main factors is the number of embryos that will be available. We recommend six normally fertilised embryos to have a reasonable chance of some reaching the blastocyst stage.

Embryo transfer will also be dependent on the timing of development of the embryos. Some embryos develop faster to blastocyst than others. Development may also be different to previous cycles. Therefore transfer may take place on any day between day 4 and day 7 after egg collection and patients must be available to have the transfer on any of these days. We will inform all patients of the progress of their embryos on a daily basis. We cannot predict exactly when the transfer will take place and we may only be able to inform you the afternoon or morning before.

IMPORTANT:
It is important to note that in approximately 50% of all cases embryos do not progress to the blastocyst stage. This means you will not have embryos to replace.

It is current unit policy to transfer a maximum of 2 blastocysts to reduce the risks of a multiple pregnancy.

Also freezing of any excess blastocysts is only undertaken for research purposes, as it is very unsuccessful at present.

If you would like to discuss any details regarding the process of blastocyst transfer, please call the Embryology Office on 0208 383 8185 (ans). IVF Unit, Hammersmith Hospital