What is the "Implantation Window"?
The ovum is fertilised in the ampulla of the fallopian tube and the fertilised egg remains in the uterus for roughly 72 hours before actual implantation. This is because there’s a specific time in which it can occur, the “implantation window”.
A reason for this window is that if implantation doesn’t happen at the certain time that it should do, i.e. the implantation window time period, it signifies that something is wrong. When there’s a risk that something is wrong, a miscarriage occurs rather than the continued development of a malformed foetus.
Implantation Cycle image courtesy of OpenLearn under a creative common licence: labspace.open.ac.uk/mod/resource/view.php?id=188463