The Wonder Weeks & Developmental Leaps - Fact or Fiction
Mothers say, “He’s going through a leap.” This phrase is whispered in dimly lit nurseries, murmured between sips of coffee at baby groups, and typed out in anxious forum posts. It refers to a developmental leap, a supposed burst of mental and neurological growth that makes a baby restless, fussy, and inconsolable. As if, overnight, something inside their small bodies shifts, disrupting sleep, intensifying their cries, and making them cling tighter to the warmth of their mother’s skin.
The concept of developmental leaps originates from The Wonder Weeks, a theory developed by Dutch researchers Hetty van de Rijt and Frans Plooij. They asserted that, at specific intervals, weeks 5, 8, 12, 19, 26, 37, 46, 55, 64, and 75, a baby’s brain experiences a leap. A transformation.
However, their research relied on the self-reported experiences of fifteen attachment-parenting mothers who were instructed on what to observe. The study in question deliberately omitted infants who naturally sleep through the night, adhere to consistent nap schedules, and demonstrate extended wakefulness while maintaining a generally content and sociable demeanor.
This selective exclusion introduces a significant research fallacy, namely selection bias, whereby the study’s findings are shaped by a predetermined subset of the population rather than a representative sample. When a study’s conclusions are presented as applying to all infants, yet the sample was curated to exclude entire categories, the integrity of the research and the conclusions are seriously compromised.
This non-scientific methodology creates the misleading impression that the observed results (from fifteen attachment parenting mothers) are universal when, in reality, they reflect only a particular subset that aligns with this specific approach to infant care. Developmental psychologists critique this approach as overly narrow. Neurologists dismiss the “leap theory” conclusion since it contradicts all established neurobiological precepts governing infant brain development.
What remains is a paradox: a theory embraced by some parents but dismissed by scientists. It might sound like a comforting narrative to those inclined to embrace it, but it does not fit into the realm of science. The brain does not adhere to a calendar, let alone specific preordained dates and weeks.
A Misinterpretation of Brain Plasticity
The Wonder Weeks proposes that babies experience cognitive breakthroughs at precise moments, i.e. specific weeks. Yet, this theory is quickly shredded by the body of neurologic evidence to the contrary. Growth is not a staircase with measured steps but an evolving landscape, synapses form, myelin sheathes nerve fibers, and neural pathways are pruned, all unfolding over months and years, not in rapid, predictable jumps.
The theory attempts to simplify what is complex. It suggests that neurological growth is linear and scheduled when, in reality, it is fluid and influenced by numerous factors beyond merely the passage of time; continuous nighttime sleep being one of the factors.
Most of the global medical community point to the flaws in Rijt and Plooij’s model starting with the fact that it fosters confirmation bias: parents look for specific signs, fussiness, clinginess, sleep disruption, all normal daily behaviors, and then interpret these normal behaviors as evidence. A restless night, additional need for comfort, surely, the baby is making a leap, or, maybe just cutting a tooth, or reacting to the Mexican food mom had last night, or, or, or! For exhausted parents, the power of suggestion becomes the lens through which they interrupt the cause of their child’s behavior.
The Anxiety Industry
There is money to be made in parental anxiety. The Wonder Weeks has grown beyond a theory, it is now a brand. Books, apps, and subscriptions are part of this profitable cycle fuelled by the fear that if a baby does not “leap” on time, something must be wrong.
Yet, when put to the test, the theory falls apart. In 2014, a study by Mathiesen and Sunde found no substantial evidence of predictable developmental leaps. Another study conducted by Carolina de Weerth, who was once Plooij’s Ph.D. student, could not replicate the findings. She measured infant behavior and tracked cortisol levels, searching for evidence of these leaps. However, none were found.
Who Is Saying What?
Several prominent organizations in neurological and developmental psychology have either expressed concerns about or refused to endorse the Wonder Weeks theory due to its lack of robust scientific evidence and bias. Although there is no official list of institutions that have explicitly rejected it, many of the most esteemed authorities in child development have chosen not to support the notion of predictable developmental "leaps."
The American Academy of Pediatrics (AAP) acknowledges that babies undergo developmental changes but does not recognize "leaps" as scientifically valid. The American Psychological Association (APA) characterizes infant development as a gradual, continuous process and has not adopted the Wonder Weeks framework. Similarly, the Society for Research in Child Development (SRCD), which adheres to rigorously tested developmental science, does not incorporate the concept of "leaps" in its research-supported models of infant growth.
In Europe, the European Academy of Pediatrics (EAP) adheres to evidence-based developmental tracking, highlighting that neural plasticity and growth are shaped by experience and environment rather than occurring in predetermined stages. The International Society for Developmental Neuroscience (ISDN) backs findings that brain development is a nonlinear, gradual process. Research on synaptogenesis, myelination, and pruning challenges the notion of sudden cognitive leaps.
To our readership, we offer this gentle warning. Do your research before embracing the next trending idea on social media! New trends and philosophies may come along, whispering promises of easier, softer ways, but not everything that is new is wise, and not everything popular is true.