Definition - What does Blastocyst mean?

A fertilized egg that has developed for five to six days is referred to as a blastocyst. The term blastocyst is particularly relevant in in vitro fertilization (IVF), when blastocysts are developed in a laboratory setting and then transferred to the uterus for implantation. This process is called a blastocyst transfer.

A blastocyst may also be referred to as a Day 5 embryo or a Day 6 embryo.

FertilitySmarts explains Blastocyst

When a sperm and egg meet, fertilization occurs. At this point, an embryo, the first stage of conception, is formed. Around 5 to 6 days after fertilization, the embryo becomes a blastocyst. The blastocyst differs from an embryo because of its advanced cell development and growth. A blastocyst contains 3 distinct features, including a fluid filled cavity and two distinct types of cells:

  1. Trophectoderm (T) cells - T cells consist of a single layer of cells around the circumference of the embryo that become the placenta and embryonic sac.
  2. Inner cell mass (ICM) - The ICM is a distinct clump of cells that form the actual baby

The amount of fluid in the cavity, along with the appearance of the T cells and ICM are the features that determine the quality of a blastocyst.

For a couple or individual undergoing IVF, it may be required to decide between an embryo transfer and a blastocyst transfer. This is a decision that is informed by the reproductive endocrinologist and embryologist based on the development of the embryos. This decision comes down to the number of days that are optimal for the embryo/blastocyst to reside in the lab before being inserted into the uterus.

For those who favor the blastocyst transfer, the timing is the most dominant factor. An embryo has been fertilized for just two to three days. With intercourse, the embryo would still be residing in the fallopian tubes, having not yet reached the uterus. With IVF, introducing the embryo to the uterus several days before it would biologically arrive is thought to be potentially disruptive to the process. Therefore, the blastocyst transfer may be considered a closer imitation of the naturally occurring biological process. Additionally, blastocyst transfer allows for a more selective choice of transfer specimens. While lab developed embryos may not have enough time to fully demonstrate their overall viability, more advanced blastocysts can. Additionally, the blastocyst transfer reduces the chances of multiple pregnancies.

Disadvantages of a blastocyst transfer include:

  • Additional expense
  • Prolonged exposure of the embryo to the conditions of a lab (versus the uterus)
  • Requirement of high-level lab equipment and capabilities
  • Potential of having no embryos to transfer
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