Blastocyst culture


  • The word ‘blastocyst’ refers to the stage that the human embryo reaches approximately five to six days after fertilization consisting 100-200 cells. The embryo must reach this stage before it can hatch and implant in the lining of the uterus.
  • Blastocysts have a large, central, fluid-filled cavity and two distinct cell types.
  • The cells in the centre of the blastocyst is called the inner cell mass and will give rise to the foetus following implantation. The single layer of cells on the outside of the blastocyst forms the trophectoderm and will give rise to the placenta.


  • Until about 10 years ago it was not possible to consistently grow (or ‘culture’) embryos to the blastocyst stage in a laboratory. Specialised commercial culture media (the fluid in which embryos are grown in the lab) have now made it possible to successfully nurture embryos to the blastocyst stage outside the human body. This laboratory process is known as ‘blastocyst culture’.
  • Blastocyst transfer simply means that the blastocyst embryos are transferred to the woman’s uterus on day 5 of embryo culture


  • Standard practice in IVF used to be to replace embryos into the uterus / Embryo Transfer following IVF/ICSI after only two or three days of development, when the embryos are at the ‘cleavage stages’ and usually composed of somewhere between two and eight cells.
  • Extending the embryo culture to five or six days gives the scientists a longer period over which to observe embryo development. The embryos are examined by embryologists to see their division and assess the quality of embryo
  • During this extra couple of days it is completely normal for only some embryos to progress and develop to form a blastocyst. Blastocysts are therefore a more select bunch, so have a greater potential to form a pregnancy than embryos at earlier cleavage stages.
  • It is also thought that transferring an embryo into the uterus at the blastocyst stage may improve its chance of implanting because we are getting the timing right. The blastocyst is the stage of development that would normally be in the uterus ready to implant. Cleavage stage embryos on the other hand would normally be in the fallopian tube, so when we put them back into the uterus they have to hang around for a couple of days before they can implant.
  • This ‘natural selection’ enables the embryologist to more accurately choose the best embryo for transfer which offers the most likely chance of pregnancy.
  • Embryo quality is important with infertility
  • With the use of blastocyst culture, a few embryos can be transferred without decreasing the overall pregnancy rate. This may reduce multiple gestations and improve human IVF outcome

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