Wonder Weeks and Baby Leaps

What does The Wonder Weeks claim?

The Wonder Weeks, the book by Hetty van de Rijt and Frans Plooij and the app built on it, tells parents that babies pass through ten "leaps" in mental development, each announced by a spell of crying, clinginess and wanting to be held, and that these arrive at ages predictable enough to put on a chart.

It rests on research the authors published as H. H. van de Rijt-Plooij and F. X. Plooij.

Where did the Wonder Weeks idea come from?

It began with chimpanzees. In a 1988 paper, van de Rijt-Plooij and Plooij described observing five chimpanzee mothers with infants under 24 months in Gombe National Park, Tanzania, building on earlier work showing that young chimpanzees "pass through distinct periods of developmental reorganization" marked by regressive behaviour.

Then came human babies. Their 1993 paper in the Journal of Child Psychology and Psychiatry describes collecting questionnaire and interview data from 15 mothers with babies aged 0 to 20 months, and reports that mother–infant conflicts "were clustered in distinct, difficult periods and were preceded by regressive behaviour of the infant". Later researchers summarise their 1992 paper as claiming ten such periods in the first 20 months.

What counted as a regression

According to Caroline de Weerth and Paul van Geert, who later set out to test it, the original study defined a regression week from weekly maternal questionnaires returned by post. Before two months, mothers had to report both more crying or fussing and more time in body contact. After two months, at least one more problem behaviour was also needed, such as less sleep, fear of other people, eating less at a meal, or difficulty with changing and dressing.

The ten periods, as originally reported

These are the weeks reported for the original study, as summarised by de Weerth and van Geert.

Has anyone replicated the leaps?

De Weerth and van Geert, at the University of Groningen, followed four mother–infant pairs weekly for 15 months, with weekly home observations of crying, fussing, smiling and body contact as well as maternal reports. Their peer-reviewed paper in the British Journal of Developmental Psychology concludes: "The results failed to support the 10-period pattern."

All four babies had temporary rises in crying, fussing and time in contact with their mothers, and all four mothers reported difficult weeks that met the original criteria. What did not appear was the timetable. In a fuller account of the study, the authors wrote that "although indications of periods of emotional instability were found, no evidence was obtained that these periods were strictly timed and followed a general pattern for all the infants."

A 1999 paper in Developmental Psychology by de Weerth, van Geert and Hoijtink, following babies weekly for 15 months, found important variability within each baby in crying, fussing, smiling and contact from 0 to 5 and 5 to 10 months, though not from 10 to 15 months. In an exploratory study of one baby observed three times a week for four weeks around her first birthday, differences between days of the same week were larger than differences between weeks in 18 of 20 comparisons.

The studies that support it

Sadurní and Rostan reported eight regression periods in the first year in 18 Catalan babies, comparable to the first eight of the original ten. A 2010 paper in the Spanish Journal of Psychology, using data from the same 18 babies, reported eight transition periods in the first year and a temporal link with the earlier regressions. Frans Plooij is a co-author of that 2010 paper.

So the evidence on each side comes from small samples: 15 babies, 4 babies and 18 babies. The Catalan work supports eight periods in the first year; the Groningen work found no fixed pattern at all. Fussy, clingy spells are well documented. A ten-step timetable that every baby follows is not established.

What is actually known about fussy weeks?

The best-measured pattern is crying itself. A 2017 meta-analysis by Wolke and colleagues, pooling 28 diary studies covering 8,690 babies, found that total fussing and crying averaged 117 to 133 minutes a day across the first six weeks and fell to 68 minutes by 10 to 12 weeks. It found "no statistical evidence for a universal crying peak at 6 weeks". The NHS describes the same arc: crying "tends to increase when they're around 2 weeks old and then gradually reduces at around 3 months old".

That matters for the early leaps. The first two periods in the original study fall at around 5 and 8 weeks, when fussing and crying are at or near their highest on average. A hard week at five weeks does not need a leap to explain it.

The same meta-analysis found colic, by the modified Wessel criteria, in 17 to 25% of babies in the first six weeks, 11% at 8 to 9 weeks and 0.6% at 10 to 12 weeks. For long bouts of crying, start with colic and excessive crying rather than a leap chart.

The same caution applies to other popular timetables: our pages on growth spurts and sleep regressions and wake windows look at what is actually published. For fussing over sleep by age, see why babies fight sleep at different ages.

Is it harmful to use the app?

Nothing in this research suggests the chart does harm, and if it helps a difficult week feel temporary, that is a real benefit. The risk is putting crying down to a leap when something else is going on. The NHS lists hunger, a dirty nappy, tiredness, wanting a cuddle, wind, being too hot or cold, boredom and overstimulation as the most common reasons for crying; check those first. Developmental milestones are also not the same as leaps; our milestones by month page covers what health services actually track.

When to speak to a health visitor or doctor

The NHS says that if your baby is crying constantly and you cannot console or distract them, or the cry does not sound like their normal cry, it can be a sign they are ill. Contact your GP or NHS 111 if they seem unwell, such as with a high temperature or no interest in feeding, and call 999 for the emergency signs it lists, including a fit, blue or grey skin or lips, floppiness or difficulty breathing. A leap is never the explanation for those. If the crying is wearing you down, the NHS says not to be ashamed to ask for help, and that you must never shake a baby.

Frequently asked questions

Are Wonder Weeks leaps real?

Fussy, clingy spells are real and common. The claim that they arrive at the same ten set ages for every baby is weaker. The original study followed 15 babies; a Groningen replication following 4 babies weekly "failed to support the 10-period pattern", while a Catalan study of 18 babies reported similar periods.

When is leap 1?

In the original study the first regression period fell at around 5 weeks. That is also when fussing and crying are at their highest overall: a 2017 meta-analysis found an average of 117 to 133 minutes a day across the first six weeks, falling to 68 minutes by 10 to 12 weeks.

Is leap 4 the hardest?

No study we found shows that one leap is harder than another. In the original study the fourth regression period fell at around 17 weeks, close to the four-month sleep changes our sleep regressions page discusses. The Groningen researchers found "important individual differences in development and no common pattern of emotional instability".

Do leaps affect sleep?

In the original study, sleeping less was one of the behaviours that could count towards a regression week after two months. The replication found difficult weeks in every baby but not at fixed ages. Our page on why babies fight sleep at different ages covers the published reasons sleep changes in the first year.