---
title: "Developmental Leaps in Babies: What the Science Says"
description: "Are developmental leaps real? What studies show about the week-by-week calendar, the link with sleep, what helps, and when to talk to your pediatrician."
canonical: https://buppi.baby/en/blog/developmental-leaps-what-science-says/
last-updated: 2026-11-05
---

# Developmental Leaps in Babies: What the Science Says

> Are developmental leaps real? What studies show about the week-by-week calendar, the link with sleep, what helps, and when to talk to your pediatrician.
>
> Canonical: https://buppi.baby/en/blog/developmental-leaps-what-science-says/

Developmental leaps are partly real. Babies do go through weeks of more crying, more holding and worse sleep, and learning something new can upset the night. What has little evidence is the calendar: the idea that every baby goes into crisis in predictable weeks.

That doesn't invalidate what you are seeing at home. It only changes what you can conclude: a hard week may have to do with a new skill, but also with illness, hunger or tiredness, and a calendar can't tell these apart.

## Where the idea of leaps comes from

The concept became known through the book *The Wonder Weeks*, by researchers Hetty van de Rijt and Frans Plooij. In a 1993 paper, they interviewed 15 mothers of babies aged 0 to 20 months and described difficult periods, with mother-infant conflict, preceded by more "regressive" behavior. Later papers cite 10 of these periods in the first 20 months.

The main support from outside the original group that we found is a 2002 study from Catalonia with 18 babies. Using the methods and instruments of the theory's authors, the researchers reported 8 periods in the first year. A second paper, from 2010, used the same 18 babies, had Plooij himself as a co-author, and found a timing relationship between these periods and developmental changes.

That is little to support a week-by-week calendar. These are small samples, with much of the information coming from the mothers themselves, who may notice more of what they already expect to see. Critics cite replication attempts that did not find the leaps, but we could not check the original papers, so we don't use them here. As far as we could find, no large study confirms the predicted weeks.

## What is well supported and what isn't

| Claim | What studies show |
|---|---|
| Babies have phases of more crying and more holding | Yes. A study that observed babies weekly for 15 months found a lot of variation in crying, fussing and time in contact with the mother, especially up to 10 months |
| Learning to crawl can worsen sleep | There is data, still from small studies |
| Each leap happens in a predictable week | Weak evidence, from very small samples |
| Every baby goes through the same leaps | Not demonstrated |
| A new skill follows each leap | One small study reports a timing relationship, without proving cause |
| The calendar shows whether development is going well | No. For that, use age milestones |

## Leaps and sleep: what can be said

The most concrete data are on crawling. An Israeli study followed 28 babies from 5 to 11 months, measuring sleep with a movement sensor. Overall, sleep kept consolidating, but there were times with more long stretches awake, and this rise was linked to the start of crawling.

In another study of 107 babies aged 5 to 8 months, those already crawling had more bedtime difficulties and more night wakings than those not yet crawling. At 7 and 8 months, crawling explained 17% of the variation in wakings. The data come from questionnaires answered by mothers at a single point in time, and the authors themselves call for more studies.

So it makes sense to expect worse nights when your baby is learning to get around. There is no way to predict the week or how long it lasts. To see what is common at each age, check [how many times babies wake at night](/en/blog/baby-waking-up-a-lot-at-night/). If sleep got worse suddenly around 4 months and your baby is otherwise well, it may be the [sleep regression](/en/blog/4-month-sleep-regression/). With other symptoms, talk to your pediatrician.

## What helps during hard weeks

These are common-sense measures, with no study testing them specifically for this period:

- **Give more floor time during the day.** Awake and with you nearby, your baby can practice rolling, sitting or crawling without having to train at 3 a.m.
- **Keep the [sleep routine](/en/blog/how-to-build-a-sleep-routine/).** The same steps give predictability, even when everything else is upside down.
- **Offer holding and comfort.** Responding during this phase is part of caring for them. If holding is weighing on you, see [how to transfer to the crib](/en/blog/baby-only-sleeps-when-held/).
- **Look at feeds and naps.** A hungry or very tired baby also gets irritable. [Wake windows](/en/blog/wake-windows-by-age/) help organize the day.
- **Take turns with another adult if you can.** Nights of little sleep weigh on you. If you reach your limit with the crying, put your baby on their back in the crib, leave the room for a few minutes and ask for help. Never shake a baby.

New skills also change safe sleep. The American Academy of Pediatrics (AAP) advises stopping swaddling when your baby shows signs of trying to roll, usually around 3 to 4 months. Keep putting them on their back, on a firm, flat surface, with no loose objects in the crib. Any medicine or supplement to calm or help a baby sleep only with guidance from your pediatrician.

## Leap calendar or developmental milestones?

If a calendar brings comfort, it is fine to use one, knowing it is an idea with little proof and no predictive value. The risk appears when "it's just the leap" delays a conversation with the pediatrician.

To follow development, [milestones month by month](/en/blog/milestones-month-by-month-0-12/) are more useful. In the revision published in 2022, the American Academy of Pediatrics and the US CDC chose milestones that at least 75% of children reach at that age, precisely to discourage a wait-and-see approach. The authors also point out that data are lacking, especially for social, emotional and cognitive milestones.

## When to talk to your pediatrician

> **Talk to your pediatrician if your baby:**
>
> - has lost something they already did, such as babbling, smiling, sitting or making eye contact
> - hasn't reached a milestone expected for their age, or if something about their development worries you, even without other signs
> - cries a lot and nothing calms them, or suddenly changes behavior
> - has been sleeping poorly for more than two weeks with no improvement (a timeframe we suggest)
> - seems to be in pain, vomits often, feeds poorly or isn't gaining weight
>
> **Seek care immediately** if your baby is very limp, hard to wake, having trouble breathing, has bluish skin, or has a fever and is under 3 months old.

Logging sleep, feeds and new skills for a few days helps you see what has changed and gives you concrete information for the appointment.

## FAQ

### Are developmental leaps real?

Partly. Babies do have phases of more crying, more holding and worse sleep, and new skills can affect sleep. What has little evidence is the idea of leaps in predictable weeks, the same for every baby. It comes from small studies, with 15 mothers in the original work and 18 babies in another study, with no large research confirming it as far as we could find.

### How long does a developmental leap last?

We found no well-established timeframe. In a study of 28 babies followed from 5 to 11 months, sleep got worse around the start of crawling while overall sleep kept improving across the period, but that doesn't set a duration for each baby. If things stay worse for more than two weeks, or come with other symptoms, it is worth talking to your pediatrician. That timeframe is our suggestion.

### Do developmental leaps disrupt sleep?

They can. Among 107 babies aged 5 to 8 months, those already crawling had more bedtime difficulties and more night wakings than those not yet crawling. In a study that measured sleep with a sensor, interruptions rose around the start of crawling. Both studies are small and don't prove it is the same for every baby.

### How can I tell a leap from an illness?

Look at the whole picture. More holding and worse sleep in a baby who is well, feeding, without fever and learning new things tend to be less worrying. Fever, pain, vomiting, refusing to feed, few wet diapers, limpness or losing something they already did should not be put down to a leap. In those cases, talk to your pediatrician.

### Is it worth following a leap calendar?

It can be a comfort, but not a prediction. Since the evidence is limited, your baby doesn't have to be in crisis that week, and a calendar should not delay an appointment. To follow development, age milestones are a better reference: the CDC and the American Academy of Pediatrics use what at least 75% of children already do at that age.