In humans, blastocyst formation begins about 5 days after fertilization when a fluid-filled cavity opens up in the morula, a ball of cells. The blastocyst has a diameter of about 0.1–0.2 mm and comprises 200–300 cells following rapid cleavage (cell division). About 1 day after blastocyst formation (5–6 days post-fertilization), which is when the blastocyst usually reaches the uterus, the blastocyst begins to embed into the endometrium of the uterine wall where it will undergo further developmental processes, including gastrulation. Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, which prevents adherence to the fallopian tube as the pre-embryo makes its way to the uterus. The blastocyst is completely embedded in the endometrium only 11–12 days after fertilization.
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Women's Health and Complications 