Spermatozoon penetrates into the oocyte (conception), day 0
Two-cell stage, day 1
Four-cell stage, day 2
Eight-cell stage, day 3
Morula (16-32 cells), day 4
Free blastocyst (following hatching), day 6
The embryo goes through its cleavage stages while it is migrating along the fallopian tube. As a blastocyst it reaches the uterine cavity at the end of the 5th day. There, the hatching and the embedding into the endometrium occur on the 6th day.