The IVF Laboratory

Once your oocytes (eggs) are removed, they are cultured in the IVF laboratory where conditions are tightly controlled.

Day 0:  Your eggs are counted and evaluated for maturity.  Typically, only mature (metaphase II, also commonly called “M2s”) eggs will accept the sperm and successfully fertilize.    Mature eggs are then fertilized via conventional insemination (the egg and many sperm are placed together in a drop of fluid) or ICSI (the injection of a single sperm into each egg).

Day 1: your eggs are evaluated for fertilization, the fusion or combining of the egg and sperm.   Remember, even with ICSI and the placement of the sperm into the egg, fertilization is not guaranteed.  Fertilized oocytes are now called embryos.

Day 2:  Your embryos are evaluated for growth and development each day.  As the embryo grows (divides) it forms additional cells through a process called cleavage.  On day 2 most normal embryos are comprised of 2-4 cells.

Day 3: Embryos at this stage usually have 6-8 cells.

Day 4: Your embryos continue to grow and develop into a compact ball known as a morula which literally means mulberry.   The embryo is comprised of about 16 cells at this stage.

Figure 2. Early Human Embryos Viewed with a Microscope
Top Row:             Left: fertilized egg • Middle: 8 cell (day 3)embryo • Right: cell adhesion
Bottow Row:       Left: morula • Middle: blastocyst (day 5) • Right: hatching embryo

Day 5:  On day 5, the embryo develops into a blastocyst, known for its fluid filled cavity.  It should have about 70-100 cells. At the blastocyst stage embryos may be transferred, cryopreserved without fresh transfer ( “freeze all” cycles), or biopsied  for pre-implantation genetic testing (PGT) then cryopreserved.

Day 6:  Embryos still undergoing cleavage will be considered for embryo cryopreservation.  These embryos may be used in future frozen embryo transfer cycles, donated to other couples, donated for research, stored in perpetuity, or discarded.