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Why is a fetus’s head disproportionately large during the third month of development?

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During the third month of development, the fetus’s head appears disproportionately large primarily because developmental advances occur first at the head end and then progress tailward. This pattern, known as cephalocaudal development, ensures that the head region gains an initial size advantage that is relinquished only very slowly throughout gestation and even into infancy.1

This early prominence is further supported by the rapid differentiation and growth of the brain. By the end of the third month, the main outlines of the brain are already recognizable, driven by the formation of three primary flexures, the differential enlargement of regions such as the cerebrum and cerebellum, and the massive growth of the cerebral hemispheres. While the fetus clearly resembles a human being by this stage, the head remains large relative to the rest of the body as the trunk and limbs continue their subsequent growth and specialization.231