Dr. Harvey Karp’s Fourth Trimester Approach (Infant Sleep in Context – Part 4)

Kathy Morelli
Jun 11, 2011
4 min read

Updated: Mar 2

In this fourth article in the Infant Sleep in Context series, we turn to Dr. Harvey Karp’s influential framework presented in The Happiest Baby on the Block.

Reframing Newborn Sleep: The Fourth Trimester

Karp shifts the conversation about infant sleep by asking parents to reconsider the nature of newborns themselves. His central premise is that the first three months of life function as a “Fourth Trimester” — a period of continued neurological maturation outside the womb. He asks parents to adjust their expectations and understand the true nature of newborns.

A four-day-old newborn is profoundly different from a four-month-old baby. Brain size, sensory processing, motor coordination, emotional regulation, and circadian rhythms are all in rapid development. Yet cultural images often lead parents to expect a smiling, interactive infant rather than a fussy, reflex-driven newborn.

He includes a quote from Dr. Arthur H. Parmalee, who says that most parents dream about receiving a baby that resembles a smiling, social, four- month-old! But instead, they, of course, receive a fussy, fetus-like newborn!

Dr. Karp considers a newborn to be very much like a fetus. In fact, he wistfully wishes that humans had pouches like kangaroos! But because we don’t have pouches, we need to learn behaviors that imitate the uterine environment in order to keep our newborn fetuses happy!

Hence, Dr. Karp believes the first three months of a baby’s life is actually a fourth trimester where a lot more maturation takes place. He suggests we need to re-learn how to parent babies in the fourth trimester.

Think about this: a newborn’s brain is 20% smaller than a four-month-old baby’s brain.

Because of this more mature brain, a four-month-old baby has more mature capabilities than a newborn. Some such capabilities are: processing sensory input faster than a newborn can, smile and interact with her parents, focus on objects across the room, and has more mature control of body movements, such as being able to roll over and move her head.

From this developmental perspective, frequent waking and intense crying are not behavioral failures — they are signs of neurological immaturity.

In contrast, a newborn has behaviors that are largely driven by survival reflexes, such as the Moro reflex and the well-known Crying Reflex. And from the study of tribal societies, Dr. Karp has rediscovered the Calming Reflex.

The Calming Reflex and the Five S’s

Dr. Karp believes the Calming Reflex evolved from conditions created in the uterus. He also posits ancient mothers needed to know how to calm their babies quickly in order to quiet them and keep the entire family group safe from predators. Thus, the Calming Reflex works quickly.

Dr. Karp says the “Calming Reflex,” is a physiological response that can be activated by recreating certain intrauterine conditions.

Drawing from both contemporary research and observations of infant care in traditional societies, he outlines five techniques — commonly referred to as the “Five S’s”:

Swaddling
Side or stomach positioning for soothing (not for unsupervised sleep)
Shushing (SHUUSSHHIINNGG)
Swinging
Sucking

When used appropriately and safely, these techniques often calm newborns quickly and effectively. The goal is not sleep training, but regulation — helping an immature nervous system settle.

Initiation of the remarkable Calming Reflex can be watched on Dr. Karp’s DVD, “Happiest Baby on the Block.” It is truly remarkable to watch Dr. Karp teach parents the Calming Reflex and to see how quickly and easily a baby falls asleep when enacting the natural human Calming Reflex. Anyone who has cared for an infant will be awestruck by the footage on the DVD.

Colic, Maturation, and Expectation

Karp also reframes colic through a developmental lens. He suggests that highly sensitive infants may simply be more reactive to early neurological immaturity. As melatonin production, body temperature rhythms, and cortical development mature around three to four months, excessive crying often decreases. Dr. Karp believes the “cure” for a colicky baby is the use of the Calming Reflex.

This reframing can be profoundly relieving for parents. Rather than assuming something is wrong with their baby — or with their parenting — they begin to see early infancy as a stage of rapid neurological growth.

Public Health Implications

Dr. Karp’s Calming Reflex works so well that he is now partnering with various agencies across the nation in order to design and implement interventions intended to reduce the sad abuse called Shaken Baby Syndrome.

Clinical Reflections

Unlike some other approaches in this series, Dr. Karp’s framework offers foundational education that benefits nearly all new parents.

Understanding the Fourth Trimester normalizes early infant behavior. It reduces self-blame and tempers unrealistic expectations. Night waking in the early months becomes a reflection of neurological development rather than parental inadequacy.

I think the fourth trimester and the Calming reflex are major discoveries and contribute hugely to parenting literature. The Five S’s are practical, accessible tools that can significantly reduce distress — for both baby and caregiver. For mothers vulnerable to postpartum depression, these strategies may offer meaningful relief by decreasing prolonged crying and supporting more consolidated rest.

While no single method resolves every sleep challenge, Karp’s developmental framing adds essential context to the infant sleep conversation.

In the final article in this series, we will explore Dr. William Sears’ attachment-centered approach to infant sleep.

References

Karp, H. (2002). The happiest baby on the block. Bantam Books.

Related in the Infant Sleep Series:

Related Areas of Support: