The process by which the egg settles in uterus )implantation( is in fact the result of the interaction between the embryo and the mother. Therefore , dealing with uterus transfiguration and also egg cell evolution after fertilization is essential.
While moving through fallopian tubes,the egg undergoes a series of mitotic divisions known as cleavage,as a result of which the number of cells increase ) the first cell division takes place 30 hours after fertilization ). These cells,known as blastomere,get smaller after each division . As cell division continues,the egg goes down the oviduct and when it gets to the 12th to 16th cell steps it assumes a new name morula )it is so called because it resembles mulberry up to this step the egg cell is still enclosed in a hyaline membrane . Around the 3rd day morula gets to endometrial cavity.
Uterus wall consists of three layers : 1 - endometrium , or the internal mucous membrane,2- myometrium,a thick flat muscular layer,3- perimetrium the lining which covers the outer portion of the uterus wall. During the settlement of the egg,the layer of uterus is in the secretion or progestrone phase)progestrone is a femal stroid hormone). At this time uterus glands and arteries become more meandering and its tissue gets thick and swollen. Following fertilization the secretional activity of glands in endometrium increases and capilaries become more winding and finally from an intricate capillary surface exactly under the lower surface of the uterus . After reaching the endometrial cavity , morula stays there in a free state for a few days , during this period it receives nutrients from the secretions of the endometrial glands.At the same time a liquid gathers up in the cell mass of morula and creates a cavity in it,as a result of which the cells are divided into two groups : the one located in the central parts are called internal cell mass;and the group located on the peripheral parts are known as trophoblast ) later , trophoblast creates placenta and its linings(.During this stage ) in the 5th and 6th days ( the process of the settlement begins and trophoblastic cells penetrate the mucous lining of uterus with the help of cell division , the action of enzymes secreted by themselves , and the readiness of the uterus ) the formation of the capillary surface under the uterus ). Gradually and thoroughly these cells gather under mucous lining of uterus ; little by little the place they entered endometrium is repaired.The egg settlement process is usually completed during the second week after ovulation.
Generally six days prior to the settlement process.The egg moves to uterus and settles in its anterior or the posterior wall ) as in the processes mentioned (.in case there is a disturbance in the transfer of the fertilized egg settlement might not occure in its normal site.Abnormal settlement of the fertilized egg away from it normal site is called out of the womb pregnancy ) ectopic pregnancy(.Most such pregnancies occur inside the fallopian tube and generally lead to abortion.