What is a blastocyst?

A blastocyst is defined as the 5th-6th day stage of an embryo’s life.

After the oocyte is fertilized, it starts its development in the fallopian tube for about three days. During this time it moves through the tube into the uterine (endometrial) cavity, where it stays for a further two – three days to develop into a blastocyst.  A blastocyst is characterized by its expansion, the thinning of the zona pellucida (the membrane surrounding the blastocyst) and the blastocoel cavity formed by the accumulation of fluid. It contains approximately 60-120 cells that form two distinctive layers: the outer cell layer (trophectoderm) that will eventually give rise to the placenta and the inner cell mass that will form the embryo itself. The blastocyst will hatch and invade the uterine lining (implantation) about 5-6 days after fertilization, in order to become incorporated into the endometrial wall for its sustainability and survival. The placenta will eventually develop and will provide for the growth of the embryo until birth.

What are the advantages of blastocyst transfer?

Recent advances in the field of human IVF, such as improved culture media, allow for more embryos to grow in the IVF lab for a longer period, up to the blastocyst stage. 

This new advancement offers advantages such as:

  • Higher pregnancy and implantation rates due to better embryo selection
  • Higher pregnancy rates because blastocyst transfer occurs closer to the natural time that an embryo enters the uterus. At this time the endometrium may provide a better environment for the embryo
  • A reduction in multiple pregnancy rates, as usually only 1-2 blastocysts are being transferred
  • The ability to assess the development of ovarian hyperstimulation syndrome (OHSS) and decide whether a fresh transfer is advisable, or whether the cryopreservation of all suitable blastocysts is safer for the patient

Is blastocyst transfer suitable for all patients?

 Transferring embryos at this stage is not recommended for all patients. Α sufficient number of good quality embryos on days 2 and 3 is required, in order to increase the chance of obtaining some good quality blastocysts on days 5 or 6.

It is also recommended when there is suspicion of developing OHSS, if previous embryo transfers on days 2 and 3 have been unsuccessful or if preimplantation genetic diagnosis is being performed.

At AKESO we have established a successful blastocyst culture system and in collaboration with the patient’s treating clinician, we will recommend it to patients if we believe they will benefit from it and increase their chances of implantation and thus establish a successful pregnancy.