In every menstrual cycle, there is an optimal time for implantation, and even a slight deviation will make things difficult. The period is estimated to be 6 to 8 days after the surge in luteinizing hormone (LH) just before ovulation. Current research indicates a special structure called “pinopod” appearing in the endometrium during the implantation window stage, and various proteins and sugars increasing, but this research is not yet complete and more information is needed. (Quinn CE, Casper RF. Human Reproduction Update, 2009, Vol.15, No.2, p229–236). As research on the implantation window progresses, we look forward to further clarification about the matter that will help us improve success rates.
Normal endometrium with regular arrangement of pinopodes [electron micrograph] Source: Color Atlas "Eggology for Infertility Treatment", "Quality of Endometrium from the Viewpoint of Thickness", Atsushi Higashiguchi