In the IVF laboratory, after the egg retrieval procedure, fertilization is carried out by IVF (In-vitro fertilization) or by ICSI (intra-cytoplasmic sperm injection) and the resulting embryos are placed in an incubator for further development. The stages of development are as follows:
The day of egg retrieval is considered as Day 0 (IVF or ICSI is done after egg retrieval)
The pronuclear stage on day 1 (embryologist checks as to how many eggs have successfully fertilised)
The two to four cell stage on Day 2
The eight cell stage on day 3
The morula stage on day 4
The blastocyst stage on day 5 or day 6
The blastocyst contains two different cell types- the inner cell mass which develops into the fetus,
and an outer layer of cells called trophectoderm which develops into the placenta.
Historically, embryo transfer following IVF or ICSI has been carried out on Day 2 or Day 3 after
egg collection. However, in the IVF laboratory advances allow us to now culture embryos for
five or six days. This gives the embryos more opportunity to prove their developmental potential.
Some embryos stop developing (arrest) prior to day 5/6. This ‘natural selection’
enables the embryologist to identify and choose the best embryo/s for embryo transfer.