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IVF clinics grade embryos at the blastocyst stage (day 5) — only well-formed blastocysts with clear inner cell masses are transferred; the rest may be frozen. Embryonic stem cells come from the inner cell mass of blastocysts — the source of regenerative medicine's promise and its ethical debates. Ectopic pregnancies often fail because the blastocyst can't implant properly outside the uterus.