If you follow our series on the first year of a child's life, you know we have been through it all—from sleepless nights to first steps. When a baby who was all smiles yesterday suddenly becomes weepy, restless and stuck to you like Velcro, many parents search for a developmental leap.
The phrase usually refers to the Wonder Weeks model, which predicts ten periods of fussiness followed by new skills during the first 20 months. This guide explains the model, its schedule and what parents commonly notice. It also separates those predictions from established developmental milestones, because a leap calendar is not a diagnosis or a timetable every baby must follow.
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What are developmental leaps?
The concept is best known from the book The Wonder Weeks by Dutch researchers Frans Plooij and Hetty van de Rijt. Their original study of 15 mother-infant pairs reported periods of crying, clinginess and reduced sleep at similar ages, followed by developmental changes.
The model proposes that each period reflects a new way of perceiving the world. It can be a reassuring way to organise observations, but the fixed schedule is the model's claim rather than a medically established pattern.
Within the model, a leap is recognised by the three C's: crying, clinginess and crankiness. Parents may also notice changes in sleep, feeding or interest in new skills. Those signs are nonspecific, however: babies can be fussy for many reasons, and ordinary milestones develop across broad age ranges.
Wonder Weeks calculations use the due date rather than the birth date. Separately, clinicians use corrected age when assessing many developmental milestones in babies born prematurely, generally during the first two years. That medical practice does not validate the ten-leap schedule, but it explains why gestational age matters when discussing early development.
What does the science say?
The fixed week-by-week calendar is not established science. A later longitudinal study designed to test the ten strictly timed periods followed four mother-infant pairs and did not reproduce the pattern. Other small studies have reported related transition periods, so the evidence is limited and mixed rather than conclusive. Use the chart as a popular parenting framework, not as proof that a behaviour has one cause or that development is on or off schedule.
Developmental leaps chart
Here is the timetable used by this guide and our calculator. These are model windows counted from the due date, not medically validated deadlines:
| Leap | Model window | Approximate age | What the model calls it |
|---|---|---|---|
| 1 | Weeks 4–6 | About 1 month | The world of sensations |
| 2 | Weeks 7–9 | About 2 months | The world of patterns |
| 3 | Weeks 11–13 | About 3 months | The world of smooth transitions |
| 4 | Weeks 14–19 | About 4–5 months | The world of events |
| 5 | Weeks 22–26 | About 5–6 months | The world of relationships |
| 6 | Weeks 33–37 | About 8–9 months | The world of categories |
| 7 | Weeks 41–46 | About 10–11 months | The world of sequences |
| 8 | Weeks 50–55 | About 12–13 months | The world of programs |
| 9 | Weeks 59–64 | About 14–15 months | The world of principles |
| 10 | Weeks 70–76 | About 17–18 months | The world of systems |
Leap by leap: what to expect
A quick reminder before the list: the leap names and week numbers come from the Wonder Weeks model. The example skills are broad age-related milestones; they do not prove that a leap caused the change or that every baby should develop on this schedule.
Leap 1 (week 5): the world of sensations
The model places the first leap around week 5 and calls it the world of sensations. Around this age, babies become more alert to faces, voices and touch, and early social smiles may begin to appear. We wrote more about this period in our article on the first month of a newborn's life.
Leap 2 (week 8): the world of patterns
The model places leap 2 around week 8 and calls it the world of patterns. At about two months, many babies look closely at faces and objects, make sounds other than crying and respond to social interaction. What that looks like from a parent's perspective, we described in our article on the second month of life.
Leap 3 (week 12): the world of smooth transitions
The model places leap 3 around week 12 and calls it the world of smooth transitions. Around three months, babies may track moving objects more smoothly, gain head and arm control and take part in longer sound-and-smile "conversations." We covered this age in our third-month guide.
Leap 4 (week 19): the world of events
The model places leap 4 between roughly weeks 14 and 19 and calls it the world of events. Around four to five months, many babies reach for objects, bring safe objects to their mouths and become more interested in cause and effect. This age can also bring changing sleep patterns, which is one reason parents often describe it as demanding. We cover the age-related milestones in our guides to the fourth and the fifth month of life.
Leap 5 (week 26): the world of relationships
The model places leap 5 around week 26 and calls it the world of relationships. At about six months, babies increasingly explore distance, movement and familiar people. Reactions to separation or unfamiliar people can also begin around this broader stage, but timing varies. More in our article on the sixth month of life.
Leap 6 (week 37): the world of categories
The model places leap 6 around week 37 and calls it the world of categories. Around eight to nine months, babies compare objects, explore textures, search for hidden things and understand more familiar words and gestures. You can read about this age in our guides to the eighth and the ninth month of life.
Leap 7 (week 46): the world of sequences
The model places leap 7 around week 46 and calls it the world of sequences. Around ten to eleven months, babies may imitate simple actions, put objects into containers and become more active participants in familiar routines. We wrote about this stage in our guides to the tenth and the eleventh month.
Leap 8 (week 55): the world of programs
The model places leap 8 around week 55, near the first birthday, and calls it the world of programs. Around this age, children often imitate everyday actions and participate in simple routines. First words and first independent steps may appear during this broad period, although both vary considerably. More in our guides to the twelfth month and months thirteen and fourteen.
Leap 9 (week 64): the world of principles
The model places leap 9 around week 64, at about 14 to 15 months, and calls it the world of principles. At this age, toddlers increasingly test cause and effect, follow simple directions, communicate preferences and experiment with boundaries. We describe the wider stage in our guide to months fifteen and sixteen.
Leap 10 (week 75): the world of systems
The model's last leap covers roughly weeks 70 to 76, around months 17 and 18. It describes a growing understanding of systems: family, home, routines, "us" and "them". Around this age, toddlers also show more independence, imitation and communication, although these are ordinary developmental changes rather than proof that a predicted leap is occurring. We described this age in detail in our article on the seventeenth and eighteenth month of life.
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See what you’ll get →How long do developmental leaps last?
There is no scientifically established duration for a developmental leap because research has not confirmed the fixed ten-leap schedule. In the Wonder Weeks model, the estimated windows range from a few weeks to about six weeks. The timing is a prediction, not an explanation for every difficult day.
How long does leap 4 last?
In the timetable used here, leap 4 spans approximately weeks 14 to 19, so about five weeks when counted from the due date. Parents often call this the 4-month leap. The window overlaps with real developmental changes around four months and the sleep regression that usually occurs in this period.
How long does leap 10 last?
The model places leap 10 at approximately weeks 70 to 76—about six weeks, ending near 18 months. It is the final predicted window in the Wonder Weeks system. Development continues after it, but the model does not define official leaps at ages two or three.
Which developmental leap is the worst?
There is no objective answer because babies and families experience difficult periods differently. Within the Wonder Weeks framework, leap 4, ending around week 19, is commonly described as one of the hardest because its model window is long and overlaps with major changes in infant sleep.
Why that one? A few reasons:
- Its model window is long. The timetable spans about five weeks, from weeks 14 to 19.
- It overlaps with changes in infant sleep. Sleep becomes organised into more distinct cycles during the first months, and some families notice more waking around this age. It is often called the "four-month sleep regression" because it can feel like a sudden change, but it is actually a gradual developmental shift that becomes more noticable at this age.
- It comes right after the "honeymoon". Around month three most babies become calmer and more predictable, so parents just start thinking "we've figured out the system". And then - boom.
The periods around leap 8 (about 12 months) and leap 10 (about 18 months) can also feel intense because they overlap with walking, teething, communication changes and a growing drive for independence.
If you are currently in the middle of the worst leap: hang in there. It passes. And on the other side, a baby who can suddenly do a whole set of new things is waiting for you.
Leaps after 18 months: 22 months, age 2 and age 3
The Wonder Weeks calendar officially ends with leap 10 around week 75. Does that mean it is all smooth sailing after month 18? Every parent of a two-year-old just laughed out loud.
Development, of course, continues - we just no longer describe it in weeks but in phases. Around month 22 many children go through an intense period of boundary testing and an ever stronger need for independence ("I DO IT MYSELF!"), which we wrote about in our article on months 21 and 22. At age two the famous autonomy phase arrives (known as the "terrible twos", though we prefer to call it the phase of the little scientist testing every single hypothesis) - all about it in our article on the two-year-old child. And at age three comes a great expansion of speech, imagination and social skills, with the occasional dramatic meltdown because the banana was cut the wrong way. More in our article on what to expect from a three-year-old.
So: leaps in the strict sense stop around month 18, but stormy periods after which the child suddenly "grows up a notch" will follow you for years. The good news is that you grow more experienced too, so nothing can surprise you anymore. (Almost nothing.)
How to survive a developmental leap?
A few things that helped us, leap after leap:
- Lower the bar. A week in which the baby will not sleep and wants to be carried constantly is not the week for spring cleaning and ambitious plans. Survival is success.
- Offer closeness and respond to cues. Carrying, cuddling and calmly checking whether your baby is hungry, tired, uncomfortable or overstimulated can help both of you through a difficult spell.
- Stick to the routine as much as you can. When the baby's inner world is in chaos, a predictable outer world means a lot.
- Offer age-appropriate play when your baby is receptive. Simple conversation, floor play and safe sensory experiences support development without needing to wait for a predicted date. Try these activities that encourage development.
- Share the load. Take turns with your partner, accept help from the grandparents. Leaps are a marathon, not a sprint.
- Do not assume every difficult spell is a leap. Fussiness can accompany ordinary development, but it can also signal hunger, discomfort, illness or another need.
Most importantly, contact your paediatrician if you are worried about feeding or weight gain, the crying is unusual or inconsolable, your baby seems ill, or your child loses a skill they previously had. A leap calendar should never delay medical or developmental advice.
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Frequently asked questions about developmental leaps
How long does leap 4 last?
The Wonder Weeks timetable used in this guide places leap 4 at approximately weeks 14 to 19, a window of about five weeks counted from the due date. That is a model estimate, not a medically established duration, and babies do not all have a fussy period on the same schedule.
Which developmental leap is the worst?
There is no objective hardest leap. In the Wonder Weeks framework, leap 4, around months four and five, is often described as especially difficult because its predicted window is long and overlaps with changes in sleep. Your baby's experience may be completely different.
When is the 18-month developmental leap and how long does it last?
The Wonder Weeks model places leap 10 at approximately weeks 70 to 76, around months 17 and 18. It calls this the "world of systems." The six-week range is a model prediction rather than a validated developmental schedule.
How can I tell my baby is going through a developmental leap?
The Wonder Weeks model describes three C's: crying, clinginess and crankiness, sometimes with changes in sleep or feeding. These signs cannot confirm a leap because they have many possible causes. Check your baby's ordinary needs first and contact a healthcare professional if the behaviour is unusual or worrying.
Are leaps counted from the birth date or the due date?
The Wonder Weeks model uses the due date. For premature babies, healthcare professionals may also use corrected age when assessing developmental milestones during the first two years. These are related ideas, but medical use of corrected age does not prove the leap calendar.
Is there a developmental leap at age 2?
According to the Wonder Weeks calendar, the last leap ends around month 18, so there is no "official" leap at age two. But development continues in stormy phases: around the second birthday the autonomy phase begins (also known as the "terrible twos"), with intense boundary testing and a strong need for independence. In intensity, it certainly feels like a leap.
Is there a developmental leap at age 3?
Formally no, but around the third birthday there is a great expansion of speech, imagination and social skills, often accompanied by strong emotions and the occasional tantrum. Many parents describe the third year as more demanding than the second, which is why people jokingly talk about "threenagers".
Do leaps last the same for every baby?
No. The model predicts the same sequence, but studies have not shown that every baby has the same fussy periods at the same ages. Development and behaviour vary widely, so use the dates as orientation rather than a strict schedule.
Are developmental leaps scientifically proven?
Babies certainly develop new skills, but the evidence does not establish ten universal periods of regression at fixed weeks. The original study reported a pattern in 15 mother-infant pairs, while a later longitudinal test with four pairs did not reproduce it. The Wonder Weeks chart is therefore best treated as a popular framework, not a clinical tool.
Sources and further reading
- Van de Rijt-Plooij and Plooij: Infantile regressions and transition periods
- De Weerth and Van Geert: Test of strictly timed developmental transitions
- Sadurní and Rostan: Regression and transition periods in early infancy
- CDC: Developmental milestones
- American Academy of Pediatrics: Corrected age for premature babies
We hope this guide helped you put the current (or upcoming) chaos into context. If you are wondering what else awaits you in each month besides the leap model, explore our month-by-month series—from the first month of a newborn's life all the way to age four.
Good luck, and remember: every leap passes. And every one brings something wondrous.
