Oocyte fertilization and zygote migration

Fertilization normally occurs in the ampulla of the fallopian tube, typically within 24 hours after ovulation. Fertilization initiates cleavage of the zygote into smaller, equipotential cells called blastomeres. The first cleavage into two cells is completed in about 30 hours, with subsequent divisions occurring at 10- to 12-hour intervals. By the end of the third day after fertilization, the structure contains 16 blastomeres, which are called the morula.
The morula enters the uterus about four days after fertilization and "floats" in the uterine cavity. During this time, it continues to divide and absorbs fluid from the uterine cavity. The resulting fluid-filled spaces coalesce to form a blastocyst cavity, at which time the morula becomes a blastocyst. A single spherical layer of trophoblast cells (trophectoderm) form the outer wall of the blastocyst while its inner cell mass becomes the embryo. The blastocyst remains free within the uterine cavity for two to four days before attaching to the uterine wall. Implantation begins seven to eight days after fertilization.