Fertilisation takes place in the fallopian tubes and the embryo enters the uterus 3-4 days later. Implantation takes place 1-3 days after the embryo enters the uterine cavity.
The endometrium is receptive to embryo only during a small window, 16-22 days of a 28-day cycle (5-10 days after the LH surge). A large number of biochemical and molecular changes is needed in the endometrium to achieve normal implantation.
In addition, the endometrium needs to undergo a series of ultrastructure changes, without which implantation would not be possible.
Without a normal receptive endometrium (uterine lining), even the best embryo would not be able to implant.
Alterations in the various biochemical factors can lead to a defective endometrium and cause the embryo to fail to implant.
Previous surgery, like treatment for miscarriage, surgical removal of fibroids, a caesarean section can lead to scarring of the uterine lining (intrauterine adhesions). This can also cause defective implantation and infertility.