First Four Days

Three to four days after conception, the morula enters the uterine cavity where it will remain unattached for about 3 days. During this time, the center of the morula fills with fluid from the uterine cavity, and as the space within develops, two distinct types of cells will form. The outer wall will develop trophoblasts and the inner cells develop embryoblasts. After further development, the trophoblast becomes part of the placenta, and the embryoblast, the embryo.