At what stage of embryonic development does an embryo normally begin to implant in the endometrium?

Ans. A BlastocystRef: I.B. Singh, 9th ed. pg. 43-44* After fertilization, the fertilized ovum begins to divide as it migrates through the uterine tube.* It undergoes cleavage till a 16 cell stage and is called morula.* Cells of morula differentiate into an inner cell mass which is completely surrounded by an outer layer of cells, which further gives rise to a structure called trophoblast.* It reaches the blastocyst stage (approximately 110 cells) on about day 5 and it enters the uterus on about day 6.* Implantation normally begins on day 6 with the syncytiotrophoblast of the embryonic pole of the blastocyst eroding into the endometrium.