What stage of embryonic development implants in the lining of the uterus?

The stage of embryonic development that implants in the lining of the uterus is the blastocyst. This structure forms after the fertilized egg undergoes several divisions and reaches the stage of about 100-150 cells, typically around 5-6 days post-fertilization. The blastocyst consists of an inner cell mass that will become the embryo and an outer layer called the trophoblast that facilitates implantation. Successful implantation marks the beginning of pregnancy.