The first division of the fertilized egg cell can take place already after approximately 22-24 hours from fertilization. After 40-42 hours from fertilization – day 2 after ovum pick-up, the embryo has 2 or 4 cells (blastomeres). This stage of division is reached by about 80-100% of properly fertilized egg cells.
After next 24 hours of culture (3 days from ovum pick-up), the embryo should reach 6-8 cell stage. This stage is reached by approximately 50-80% of 4-cell embryos.
The next 24 hours of culture (4 days from ovum pick-up) lead to the stage of the so-called morula. Then, it is already difficult to assess the exact number of cells in the embryo. The embryo most often looks like a homogeneous cell mass. Typically, only 20-40% embryos reach this stage.
After another 24 hours of culture (5 days from ovum pick-up), the embryo enters into the last stage of development possible in laboratory settings – a blastocyst. It is the most advanced stage of embryo development. Only the strongest embryos reach this stage.