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Physical growth is a fundamental feature of an infant’s first year, evident as the average neonate triples their weight and becomes 50% taller, rapidly outgrowing clothing while uttering their first words, enduring eruption of their first teeth, and taking their first steps. This remarkable transition in form continues a journey that began roughly 270 days earlier (Jukic, Baird, Weinberg, McConnaughey, & Wilcox, 2013) when a 0.5 micron fertilized egg cell initiated a series of differentiation, proliferation, and expansion events. Within days of consolidating the genetic material from maternal and paternal germ cells, the new zygotic genome becomes activated (Braude, Bolton, & Moore, 1988), replacing a sole reliance on proteins from the mother’s egg and reproductive tract. The first cell undergoes successive mitotic cycles resulting in cell proliferation within the original membrane formed by the fusion of the sperm and egg. By the end of the first week, the contiguous mass of new cells is compressed by the emergence of a fluid-filled cavity, creating an inner cell mass within the membrane.