4th weeks
- Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form theplacenta and embryonic membranes. The blastocyst is fully implanted day 7–12 of fert.
- Formation of the yolk sac.
- The embryonic cells flatten into a disk, two cells thick.
- If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined.
- Primitive streak develops. (day 13 of fert.)
- Primary stem villi appear. (day 13 of fert.)