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A blastocyst is a microscopic group of about 150 cells — smaller than Roosevelt’s eye on the face of a U.S. dime. At day five after fertilization, when stem cells can be harvested, a blastocyst is approximately 0.1 to 0.2 millimeters (0.0039 to 0.0079 inches) in size – about the size of the period at the end of this sentence.
