Stage 4 is reserved for the attaching blastocyst that is adhering to the endometrial lining of the uterus. The attaching process is called adplantation and heralds the onset of implantation. Stage 4 embryos have an estimated postfertilization age of approximately 6 days.
The outer trophoblast layer of the hatched blastocyst has multiple areas of attachment to the endometrial epithelium. In the areas of contact the syncytiotrophoblast dissolves the endometrial epithelium. The epithelial cytoplasm disappears before the nuclei, resulting in an accumulation of nuclei in these areas. Progressive differentiation of the trophoblast is taking place. The cytotrophoblast becomes distinct from the overlying syncytiotrophoblast. Amniogenic cells are separating from the trophoblast and are distinct from the inner cell mass. The hypoblast layer is a distinct, thin layer on the underside of the inner cell mass. The remaining cells constitute the epiblast.