Blastocyst Culture

IVF, In Vitro Fertilization with Blastocyst Culture and Day 5 Transfer

  • Blastocyst culture and day 5 embryo transfer for in vitro fertilization allows selection of the best quality embryos for transfer
  • Embryo quality is important with infertility
  • With blastocyst transfer, we transfer fewer embryos – reducing multiple pregnancy risks, and maintain high IVF success rates

Definition of a Blastocyst

  • An embryo that has developed for five to seven days after fertilization and has 2 distinct cell types and a central cavity filled with fluid (blastocoel cavity)
  • The cells in a blastocyst have just started to differentiate
  • The surface cells that surround the cavity (just under the outer shell) are called the trophectoderm and will later develop into the placenta
  • A more centrally located group of cells – the inner cell mass, will become the fetus

Blastocyst Development

  • The blastocyst usually forms on day 5 as fluid builds within the compacted morula
  • A healthy blastocyst often begins hatching from its outer shell, called the zona pellucida between day 5 to day 7 after fertilization
  • Within 24 hours after hatching, embryo implantation after IVF (or a “natural” pregnancy) begins as the embryo invades into the uterine lining
  • The blastocyst releases HCG hormone (the pregnancy test hormone) which leaks into the mother’s blood as the embryo implants

Blastocyst Formation Rate

  • The goal of in vitro fertilization and embryo culture is to provide high-quality embryos which are capable of continued development and result in live births
  • However, under standard IVF culture conditions, only about 25 to 60% of human embryos progress to the blastocyst stage after 5 days of culture
  • The low rate of embryo development has 2 main causes:
    • A less than optimal culture environment in the lab dish
    • The inherent “weakness” of human embryos
  • Therefore, in the past embryos were transferred to the uterus after 2 or 3 days

Advantages of blastocyst transfer for IVF

  • One problem with this is that 2 to 3-day-old embryos are normally n the fallopian tubes, not in the uterus. The embryo gets to the uterus about 80 hours after ovulation.
  • Embryo implantation process begins about 3 days later – after blastocyst formation and hatching out of the embryonic shell have occurred.
  • Therefore, if in vitro culture conditions are maximized so healthy blastocysts form at a high rate, then day 5 blastocyst embryo transfer can be done.
    • The uterine lining on day 5 should be receptive to the arriving embryo – this a more “natural” time for the embryos to be in the uterus. It is the same timing as with a natural pregnancy.
    • The transfer is done shortly before the time for actual invasion and implantation

Maintain pregnancy rates and reduce risks for multiples

  • Transferring blastocysts following IVF also provides another potential benefit – reducing possibility for multiple pregnancies.
  • Many 2 or 3-day-old embryos do not have the capacity to become high-quality blastocysts and make a viable pregnancy. However, on day two or three of culture we don’t have methods to determine which embryos will be viable long-term, and which will soon arrest their development.
  • By culturing embryos to day 5 we will find that some of them have not become blastocysts – allowing us an opportunity to choose the most competent embryos for transfer.
  • We can then transfer fewer embryos and still obtain high pregnancy success rates – with very little risk for having a high order (triplets or higher) multiple pregnancies.

Extended in-vitro culture systems

  • In the past, it was difficult to get high-quality blastocysts with in vitro culture systems – unless “feeder” cells were utilized – called coculture.
  • However, since 1998 more advanced culture media have been commercially available that (if used properly) can yield high blastocyst formation rates.
  • Now blastocyst embryo transfer is a viable IVF treatment option for many couples

Disadvantages of blastocyst transfer

  • If the IVF clinic is proficient at blastocyst culture and has excellent quality control in the laboratory, there is potentially no disadvantage to day 5 blastocyst transfer.
  • However, if the culture environment is suboptimal, delayed embryo development and even embryonic arrest will occur in some cases.
  • Therefore, if the culture system and laboratory quality control are inconsistent – good results will not be obtained with extended culture to day 5.
  • Such programs will do better with day 3 transfers – putting back embryos earlier before they are “stressed” excessively by the weak culture environment.
  • Another possible disadvantage – some clinics charge more for day 5 transfer

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