It generally takes 6 to 7 days for a fertilized egg to form and become fertilized. After ejaculation, the viable spermatozoa pass through the vagina, cervix and uterine cavity to the fallopian tube, where the spermatozoa and secondary oocytes combine to form a fertilized egg. Afterwards, the fertilized egg moves towards the uterine cavity under the impetus of peristalsis and epithelial cilia in the fallopian tube, and undergoes mitosis at the same time, gradually developing into the stage of mulberry embryo, early blastocyst, and late blastocyst. Early blastocysts enter the uterine cavity around the 4th day after fertilization, and early blastocysts disappear the zona pellucida on the 5th~6th day after fertilization and divide and develop into late blastocysts. Around 6~7 days after fertilization, late blastocysts implant into the endometrium through three processes: localization, adhesion and invasion to complete the implantation. The completion of implantation requires not only the development of the fertilized egg, but also the secretion of sufficient estrogen and progesterone to support the function of the mother’s uterus, and the simultaneous development and coordination of the blastocyst and the endometrium.