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The early child period is considered to be the most important developmental phase throughout the lifespan.
What weeks of development does the pre-embryonic period include? The pre-embryonic period is the first 2 weeks of development (the first 2 weeks after fertilization), when the single cell produced by fertilization (the zygote) becomes a spherical, multicellular structure (a blastocyst). This period ends when the blastocyst implants in the lining of the uterus.