Last Updated: May 9, 2018

Definition - What does Blastocyst mean?

A blastocyst is a fertilized egg that has developed for five to six days and contains 3 distinct features. These features include 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

During in vitro fertilization (IVF), blastocysts are developed in a laboratory setting and then transferred to a uterus, hopefully resulting in implantation and pregnancy.

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, a zygote is formed. Cells continue to divide and around 5 to 6 days after fertilization, the embryo becomes a blastocyst. A blastocyst differs from an embryo because of its advanced cell development and growth. This includes a fluid-filled cavity and two distinct types of cells: T cells, and the ICM. Around 5 or 6 days after fertilization, a healthy blastocyst will hatch from its protective outer shell known as the zona pellucida. It is around 24 hours after this hatching process that embryo implantation is ready to occur.

Blastocyst DiagramImage caption

Having high-quality embryos to transfer to a uterus is the most important part of the IVF process. Blastocysts are graded by an embryologist to determine which have the greatest likelihood of resulting in pregnancy. 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 a number of factors including 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 or by the overall number of embryos that continue to develop.

With a blastocyst transfer, the timing is more similar to what would occur through natural conception. An embryo has been fertilized for just two to three days. With conception through intercourse, the embryo would still be moving through 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, a 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. While lab-developed embryos may not have enough time to fully demonstrate their overall viability, more advanced blastocysts can. Additionally, because a more informed decision can be made, fewer blastocysts are typically transferred which reduces the chances of multiple pregnancies.

Disadvantages of a blastocyst transfer can include:

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