After cellular division to 64 cells, the morula becomes a blastocyst, a more compact and solid mass.
Once formed, the blastocyst moves freely in the uterine cavity for 48 hours before finding a place to implant itself in the endometrium.
The endometrium relaxes to ease implantation of the blastocyst. Nine days after fertilization, the embryo will already be in the uterine wall.
After mplantation, the embryo begins to grow. If the woman has very low levels of estrogen and progesterone, the endometrium can rupture and cause implantation to occur in the wrong place.