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Blastocyst embryo transfer

The first IVF human pregnancy was achieved by blastocyst transfer. Blastocyst transfer is claimed to be more physiological than pronucleate or cleaved-embryo transfer is as it mimics nature more closely. As the embryo advances in the development, after 5-6 days it becomes a blastocyst. This has an outer thin layer of cells which will later form the placenta, and an inner cell mass which will develop into the fetus. A blastocyst has about 120 cells.

The structure of a blastocyst.

A blastocyst gives a better idea of the competence of an embryo and has a higher chance of implantation than a cleaved embryo.

In conventional culture medium, about 20% of embryos will develop into blastocysts. Recently, the use of sequential culture medium (the embryos are cultured in different media according to their stage of growth) has enabled a larger number of embryos to develop into blastocysts. However, up to 40% of patients will not grow blastocysts and will not have blastocyst embryo transfer.

The rational behind a blastocyst transfer is that an embryo which has failed to reach the blastocyst stage, would be unlikely to have resulted in a pregnancy. However, if it reaches the blastocyst stage it has about 50% chance of implanting. So the improved implantation rates following blastocyst transfer is due to selection of the best embryos.

Why then do 50% of the blastocysts fail to implant?

A defective blastocyst (e.g. chromosomal abnormalities) is a possible cause; a non-receptive endometrium is another cause.

Blastocyst embryo transfer into the uterine cavity is performed about 5-6 days after egg collection. Transfer of one or two blastocysts is recommended to avoid high-order multiple pregnancies. Supernumery blastocysts can be frozen for future use.

For whom blastocyst transfer is recommended?

  • Patients who had repeatedly failed to achieve a pregnancy following the transfer of good quality cleaved embryos (If the embryo arrest and did not develop to blastocyst, this may indicate a potential egg problem)

  • Patients who wish to achieve a pregnancy without the risk of multiple pregnancy, here the transfer of one blastocyst will be recommended.

  • Patient who do not wish to have their spare embryos frozen for whatever reasons may be advised to have blastocyst transfer.

What are the disadvantages of blastocyst trasnsfer?

  • About 10% of the embryos that fail to develop to blastocyst in vitro may have done so if replaced inside the womb on day 2 or 3
  • Up to 40% of patients will not have blastocyst available for transfer
  • Freezing spare blastocyst is not as good as freezing cleaved embryos.

What is the live birth rate following blastocyst embryo transfer?

Age group less than 35 35-37 38-40 41-42 43 or over
Blastocyst transfer 44.3% 37.6% 29.4% 18% 7.5%
Cleaved embryo transfer 37.6% 32.7% 24.4% 14.3% 5.7%

Live birth rate per treatment cycle using blastocyst transfer. Data adapted from SART report 2000

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