The journey from egg to blastocyst
In natural conception, the open end of the fallopian tube moves towards the ovary. At ovulation the egg is released when the surface of the follicle gently tears open.
The open end of the fallopian tube is fringed with little tentacles (a bit like a sea anemone) and these little fronds, called fimbriae, gently catch the egg as it is released and waft it into the open end of the tube. Microscopic hairs create gentle waves of current, moving the egg slowly deeper into the tube, and there it waits to be fertilised. If it’s the lucky egg, sperm will be making their way towards it. Only the fittest sperm will reach the egg, as they will have had to swim from the vagina, into the cervix up into the uterine cavity, across the cavity and into the tubal opening, then along the length of the tube. The first sperm to reach the egg will penetrate it, and the egg responds by locking out any other sperm which might reach it.
The day after this fertilisation process, the egg and sperm join their genes together and the embryo then starts to grow by cell division, first two cells, then four, then eight and so on as each cell splits into two. During this process of cell division the tube continues to gently waft the embryo towards the uterus, where it arrives about five days after fertilisation. By this time the embryo has a few hundred cells, and is called a blastocyst.
When IVF was first developed embryos were transferred at the early cleavage stages (between two and eight cells) as it was not possible to culture embryos successfully in the laboratory beyond this stage. Eventually the Embryology Scientists discovered the secrets of culturing embryos up to blastocyst stage (Day five), as the very exacting requirements of embryo growth were revealed. It was then possible to mimic nature more precisely by putting blastocysts into the uterine cavity, which is when they would normally be expected by the endometrium awaiting their implantation. This is one of the reasons why blastocyst transfer allows a higher pregnancy rate. The other important reason is that an embryo which can achieve blastocyst status is a very fit and active embryo, and has a higher implantation potential.
What is a blastocyst?
A blastocyst is an embryo that has been allowed to develop for five days or more after egg collection. By day five it has developed a small cavity which contains a clump of cells that will grow to form the baby. This clump of cells is called the inner cell mass. The outer cells form the placenta and membranes.
Why doesn’t everybody have blastocyst transfer?
Only a proportion of cultured embryos reach blastocyst development stage, and many of the early embryos may falter and fail to grow further. Blastocyst culture therefore is a more selective process, enabling the embryologists to select the “cream of the crop” to give the very best chance of a successful pregnancy. If it becomes evident which is the best embryo at an earlier stage, or if there are only one or two embryos, they will be transferred at that point.
At Bath Fertility the majority of our patients have embryos transferred at the blastocyst stage.