Implantation

Fertilization usually occurs in the ampulla of the fallopian tube. The resulting zygote divides repeatedly while travelling toward the uterus. These early cleavage divisions increase cell number without increasing the overall size of the conceptus, which remains enclosed by the zona pellucida. By about 3–4 days after ovulation, the conceptus enters the uterine cavity as a morula. Fluid then enters the cell mass, creating a blastocyst. The inner cell mass will form the embryo and supporting embryonic structures; the outer trophectoderm will contribute to the placenta. Before attachment can occur, the blastocyst must hatch from the zona pellucida. The zona protects the early conceptus during tubal transport, but it would also prevent direct contact with the uterine lining if it remained intact. A hatched blastocyst can finally interact with the luminal epithelium of the endometrium. This sequence explains why implantation cannot occur immediately after fertilization. The conceptus needs time to reach the uterus, develop into a blastocyst, and shed its protective outer shell.