The “4th Trimester”: Why Your Baby is Still in Womb Mode

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American pediatrician, Dr. Harvey Karp, is often credited with coining the term “4th trimester” to describe the first three months after a baby is born. Dr. Karp's book, The Happiest Baby on the Block, discusses the importance of this period for both the baby and the parents.

Human babies are born remarkably immature compared to other mammals, primarily because of their exceptionally large brains. As the human brain evolved to become more complex and capable, it also grew in size, creating a unique challenge during childbirth. To safely pass through the birth canal, human infants must be born before their brains—and bodies—are fully developed. This early birth means babies enter the world helpless, relying completely on caregivers for survival and requiring months or even years of postnatal development before achieving basic independence.

This is where the concept of the "fourth trimester" comes in: the first three months after birth are considered an extension of gestation, during which babies are adjusting to life outside the womb. During this period, they need a womb-like environment to support their continued neurological and emotional development.