You are here: Home  Treatment options  Blastocysts


A fertilised egg changes very quickly over the first few days and is called a zygote during this period. After just 24 hours, the fertilised egg cell divides into two cells. On the second and third day, these split up again to form four then eight cells. After the fourth day, the embryo already contains 16 to 32 cells. The zygote resembles a blackberry during this time, which is why this phase is known as the berry or morula stage. On the fifth day after fertilisation the blastocyst forms. It is the first form of the embryo in which different cells can be recognised.

The blastocyst still grows inside the original egg shell, with a small cyst in the centre. The different zones of the blastocyst are divided into three sub-areas:

  1. Inner part (embryoblast) – from here the baby will develop
  2. Outer part (trophoblast) – from here the placenta will form
  3. Inner cavity (the fluid inside the blastocyst) – this is the subject of intense research. Attempts have been made to draw conclusions about the genetic makeup of the embryo via the fluid inside or via the fluid in which the blastocyst grows (NI PGT-A)

The stage of development of the blastocyst is of particular interest during artificial insemination because during this time the embryo is about to implant into the uterus. The blastocyst brings a pregnancy rate twice as high as an embryo transfer on day 3.

In Linz, our embryologists divide the blastocyst quality into different grades:

  • The first number refers to the size (1 to 4)
  • The first letter assesses the internal cell dimensions (A to D)
  • The second letter assesses the outer cell dimensions (A to D)

A very good ‘expanded blastocyst’ therefore has the grade 4AA. The chances of pregnancy increase as the quality of the embryos increases. However, this does not mean that you can only get pregnant with perfect embryos. A little luck is necessary too!

With 4AA blastocysts, a single embryo transfer is generally made in order to avoid twin or multiple pregnancies. Many studies show that it is better to use only one perfect embryo than two embryos of poor quality.