Figure 1: Description of the about one week-long journey of a zygote from ovulation via fertilisation to nidation in the uterus.

A short description of the journey, about one week, of a zygote from ovulation, via fertilisation, to nidation in the uterus:

Day 1: Shortly before ovulation, the oocyte completes the first maturation division. As soon as the spermatocyte penetrates the oocyte, the second maturation division is completed. In the oocyte, female and male genotypes are identifiable as so-called pro-nuclei. Fertilisation is completed once the pro-nuclei have fused and the zygote starts to divide (cleavage division). From this point on the term "embryo" is used.

Days 2-4: During passage through the fallopian tube, the divisions continue and there is a gradual progression from four, over eight, to multi-cellular stages.

Days 5-6: At a cell count of between 32 and 58, the blastocyst starts to form by differentiating the outer cells into the surrounding trophoblast and the inner cells into the embryoblast (inner cell mass).

Days 7-9: Before the blastocyst is implanted in the endometrium, it leaves the protective zona pellucida, a process which is called "hatching".

Further background on embryonic development is presented e.g. here: