NORMAL EMBRYOLOGY OF THE FIRST TRIMESTER

The blastocyst stage of the conceptus is present late in the first week post-fertilization, prior to implantation. The blastocyst consists of the inner cell mass and the outer trophoblast.

THE BLASTOCYST

Inner Cell Mass which will later form the embryo and most of the extraembryonic membranes.	Outer Trophoblast which will form the fetal-maternal interface.