Brain boost
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Your baby’s brain is adding billions of new nerve cells. His senses of hearing, smell, and touch are developed and functional. During the third trimester, the brain triples in weight, and the cerebrum develops deep, convoluted grooves that provide extra surface area without taking up more room in the skull.
Nerve protection
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A protective covering of myelin begins to form around his nerves during this trimester, a process that will continue for a year after he’s born.