• Near the end of the first week, the blastocyst implants in the posterior wall of the uterus in the presence of high hormone levels; the blastocyst attaches at the embryonic pole (inner cell mass) so the embryo will eventually be attached dorsally. Occasionally, the blastocyst will implant on another spot in the uterus, potentially causing problems (see placenta previa).
• The trophoblast layer undergoes mitosis upon contact with the uterine wall and differentiates into the cytotrophoblast ("cellular" layer) and the syncytiotrophoblast ("multinucleated" layer).

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