What is a Blastocyst?

Normally, when a woman ovulates, her egg is sucked into the fallopian tube. During coitus, millions of sperm are deposited in the vagina and swim their way towards the egg along the uterine cavity and the fallopian tubes. Fertilization then happens when a sperm penetrates and fuses with the egg to form an embryo. The embryo then grows every day through a process of cell division. It first becomes a cleavage stage embryo on days 2 and 3 in the fallopian tube and then becomes a Morula on day 4. On day 5 it becomes a Blastocyst. As it grows, it travels along the tube towards the uterus and finally reaches the uterus in the Blastocyst stage and implants in the endometrial lining of the uterus.

What is Blastocyst culture and transfer?

In an IVF cycle, fertilisation and embryo culture take place in the lab and the embryos are transferred into the uterine cavity for implantation.

An advanced stage day 5 or 6 embryo is called a Blastocyst. At this stage, the blastocyst has two distinct cell lines, called the Inner Cell Mass (ICM) and the Trophectoderm. The ICM contains cells that eventually give rise to the foetus whereas the trophectoderm will form the key cells of the placenta.

Blastocyst transfer is a strategy in which embryos are cultured to the blastocyst stage before being transferred into the womb.

Blastocyst grading or quality is determined by evaluating the external appearance of the blastocyst by the Embryologist. The best quality blastocyst is then selected for transfer into the uterus.

How is the Blastocyst transfer done?

Embryo transfer – A soft flexible catheter containing the Blastocyst is gently inserted through the cervical canal into the uterine cavity and deposited there under ultrasound guidance. The catheter is then gently removed and the embryologist will check the catheter to ensure that the blastocyst has not been retained in the catheter. Once delivered into the uterine cavity, the blastocyst starts to attach to the uterine lining in a process known as implantation.

What are the advantages of Blastocyst transfer?

Not all embryos have the potential to develop into a blastocyst, either in the womb or in the laboratory.

Growing embryos to the blastocyst stage help to identify which embryos may be stronger than the others and that have good potential to generate a pregnancy.

Therefore, blastocyst culture is a valuable tool to facilitate the selection of the best embryos that would be most likely to produce a pregnancy.

By allowing extended culture for several days, we will be able to ‘de-select’ non-viable embryos.

Blastocysts of good quality are transferred or cryopreserved for future use.

Higher pregnancy rates are associated with blastocyst transfer as compared to a cleavage-stage embryo, as this is the stage in which embryos reach the uterus for implantation even in a natural cycle.

Fewer blastocysts (1 or 2) are transferred as compared to the cleavage stage in which usually 2-3 embryos are transferred, as Blastocysts are in the advanced stage and hence the risks for multiple pregnancies are lowered.

What are the disadvantages of Blastocyst transfer?

The ‘disadvantage’ is that fewer embryos will survive or grow to the blastocyst stage. Sometimes, there is a possibility that there may be no available embryos for transfer, as embryos may get arrested in the cleavage stage and fail to become a blastocyst.

Such embryos do not give rise to pregnancy and hence are not worth transferring. They are discarded after informing the couple and taking their consent.

Is Blastocyst transfer for me?

Blastocyst-stage extended culture and transfer should be considered if:

  • You have produced good quality embryos in a previous IVF cycle but they failed to implant in the womb.
  • You are concerned about the risk of multiple pregnancies and wish to have only one embryo transferred.
  • You have several good-quality embryos on Day 3.
  • You have explored other treatment options and want to assess the developmental potential of your embryos.
  • You are going for PGT in an IVF cycle.