Newborn to Toddler: Growth, Sleep, and Mental Leaps

Somewhere between the exhausted newborn days and your child’s second birthday, an enormous amount of brain and body development happens. Your baby goes from someone who needs help with absolutely everything to a toddler who can walk, talk, feed themselves and boss you around a bit. It can feel like a blur, especially when you’re in the thick of a growth spurt or a run of terrible nights.

One popular way parents try to make sense of the fussy stretches is the “Wonder Weeks” framework, based on research by Dutch scientists Frans Plooij and Hetty van de Rijt. It maps out roughly ten “mental leaps” in a baby’s first 20 months, each one linked to a burst of brain development that can show up as clinginess, crying, poor sleep or general crankiness. It’s worth saying upfront that this is one framework, not medical gospel. Every baby develops on their own timeline, and plenty of babies don’t fit neatly into these week-by-week windows. Use it as a rough guide to what might be going on, not a schedule to worry about if your baby is a week or two “behind.” If you ever have genuine concerns about your baby’s growth, feeding or development, your health visitor, GP, midwife or paediatrician is the person to talk to, not a chart.

With that in mind, here’s what tends to happen in the first year and a bit, physically and mentally, and what you can actually do about it.

Weeks 1 to 4: The Fourth Trimester

The first three months are often called the fourth trimester, because in many ways your baby is still finishing development that would have continued in the womb. Their nervous system is immature, their stomach is tiny, and they genuinely have no concept of day and night yet.

During these early weeks, feed on demand rather than by the clock. Newborns often feed 8 to 12 times in 24 hours, and breastfed babies especially tend to cluster feed in the evenings. Growth spurts commonly turn up around 7 to 10 days, 2 to 3 weeks, and 4 to 6 weeks, though the exact timing varies a lot from baby to baby. During a spurt you might notice your baby wanting to feed much more often for a day or two, seeming unsettled, or waking more at night. This is normal and it’s their way of telling your body (if breastfeeding) to make more milk, or simply meeting the needs of a rapidly growing baby. It usually settles within a few days.

What’s not normal: a baby who is losing weight, has very few wet nappies (fewer than 6 a day once feeding is established), or seems lethargic rather than just fussy. Those are reasons to call your midwife, health visitor or GP promptly rather than waiting it out.

Around Week 5: Leap 1, “Changing Sensations”

Just as you’re starting to feel like you’ve got a handle on your baby’s patterns, things shift again. Around 5 weeks, many babies seem to become more aware of sensations beyond just hunger and discomfort, sounds, light, touch, all of it registering more sharply. This can coincide with the peak of colic-type crying for some babies, which typically eases by around 3 to 4 months. If crying is prolonged, high-pitched, or your baby seems in pain rather than just unsettled, get them checked over rather than assuming it’s “just a leap.”

6 to 7 Weeks: Another Growth Spurt

Expect another stretch of increased feeding and possibly disrupted naps around this age. It often catches parents off guard because it comes so soon after the last one. Keep offering feeds on demand and try to rest when you can, this one tends to pass within a few days.

Around Week 8: Leap 2, “Patterns”

By about 8 weeks, babies often start noticing patterns, both in the world around them and in their own bodies. You might see your baby staring at their hands, kicking their legs rhythmically, or making little strings of sounds and seeming to listen to themselves. Some become briefly fascinated by shadows, ceiling fans or the pattern on a blanket. This is a genuinely nice stage to watch, it’s early evidence of a brain starting to organise information.

9 to 11 Weeks

Still firmly in the fourth trimester. Try not to watch the clock too closely, some babies feed frequently, others go longer between feeds, and both can be completely normal. This is a good window for regular tummy time (a few short sessions a day, always supervised) to build the neck and shoulder strength they’ll need for rolling later on.

Around Week 12: Leap 3, “Transitions”

Around 3 months, babies often become more sensitive to gradual changes: a dimming room, a voice trailing off, a shifting light. You might notice more physical restlessness than obvious “mental” signs at this stage. Some babies get more easily overstimulated in busy or bright environments during this window, so a quieter, calmer setting can help if they seem fussier than usual.

Weeks 13 to 18: Two Growth Spurts Back to Back

Most babies go through a growth spurt around 3 months and another around 4 months. Sleep very often takes a hit during this stretch, and it has nothing to do with anything you’re doing wrong.

A Note on Sleep Regression

The 4 month mark deserves a proper explanation, because this is the one sleep disruption that has a real, well understood physiological cause. Around this age, your baby’s sleep architecture matures and starts to look more like an adult’s, cycling through lighter and deeper stages rather than one long undifferentiated sleep. That means more brief wake-ups between cycles, and unlike a newborn, your baby is now alert enough to properly notice if something’s different (like waking up somewhere other than where they fell asleep). This particular regression tends to stick around rather than disappear in a few days, which is why so many parents remember it vividly.

At other ages, the picture is less clear cut scientifically. Many parents and sleep consultants notice that unsettled nights often line up with a new physical skill (rolling, crawling, standing), teething, separation anxiety, or a developmental leap, and babies genuinely do seem to practise new skills in their sleep, you’ll sometimes see a baby attempting to roll or crawl while still half asleep. Whether that’s the direct cause of the wake-up or just a coincidence isn’t something researchers can say with certainty. Either way, the practical advice is the same: keep bedtime routines calm and consistent, stick to safe sleep guidelines (back to sleep, clear cot, room sharing for the first 6 months), and know that these rough patches are usually temporary. If poor sleep goes on for weeks with no let up, or you’re worried about your own wellbeing from lack of sleep, talk to your health visitor.

Around Week 19: Leap 4, “Events”

For your baby, absolutely everything is new. You know a dropped ball will bounce. You know someone who jumps will land again. Your baby is only just starting to work out that the world runs on rules like this. Having built up an understanding of patterns and transitions in the previous leaps, they now start to grasp short sequences of cause and effect, dropping a spoon and watching what happens is a favourite “experiment” around this age (and yes, they will want to do it fifty times).

Weeks 20 to 24

Physical skills are coming thick and fast now. Many babies start rolling in both directions somewhere in this window, and some are beginning to sit with support. This is also roughly the age when solid foods usually enter the picture. Current UK guidance (NHS) and WHO advice is to start introducing solids at around 6 months, alongside continued breast or formula milk. Signs of genuine readiness include being able to sit up with support and hold their head steady, good hand-eye coordination (reaching for and grabbing food), and no longer automatically pushing food back out with their tongue. Starting much before 4 months isn’t recommended. If you’re unsure whether your baby is ready, or have questions about allergenic foods like egg or peanut, your health visitor or GP can talk you through current guidance.

Around Week 26: Leap 5, “Relationships”

Around 6 months, babies start to grasp relationships between things, and the biggest one is distance. Suddenly the world feels much bigger and slightly more alarming. A baby who can’t yet move independently finds anything beyond arm’s reach genuinely out of their control, so if you leave the room, it can feel to them like you’ve vanished entirely. This is often when the first signs of separation anxiety show up, linked to a growing understanding of object permanence (the idea that things, and people, still exist even when out of sight).

Weeks 28 to 36

Another growth spurt often lands in this stretch, so don’t be surprised by more night waking. This is also prime time for crawling to emerge for a lot of babies, though it’s worth knowing that a good number of babies skip crawling altogether and go straight to pulling up and cruising. Both are completely normal. Stranger anxiety often ramps up around 8 to 10 months too, which is a healthy sign your baby has formed a strong attachment to their main carers, even if it makes handovers at nursery or with grandparents a bit tearful for a while.

Around Week 37: Leap 6, “Categories”

By this point your baby becomes a small, dedicated researcher. Expect a lot of picking tiny things up off the floor and inspecting them (and yes, trying to taste them, so this is a good time to double check your floors for choking hazards). Squishing food, tasting different textures, sorting toys by size or shape, all of this is your baby building mental categories, working out what belongs with what. The mess is annoying but it’s genuinely productive brain work.

Week 40, 9 Months

Another growth spurt, another round of unsettled nights. Babies at this age are often becoming much more mobile, so this is also a sensible checkpoint to look again at babyproofing: stair gates, securing furniture that could tip, and getting plug sockets covered if you haven’t already.

Around Week 46: Leap 7, “Sequences”

Where the last leap was all about pulling things apart, this one is about putting them back together. Your baby starts to understand that things happen in a certain order, stacking rings on a peg, putting a lid on a box, copying a simple two step action they’ve watched you do. It’s a good stage for simple cause and effect toys and copying games like waving bye bye or clapping.

Week 48

Another common stretch of broken nights. Teething is often a factor around this age too, with molars sometimes starting to come through, so it’s not always easy to separate a developmental leap from simple teething discomfort.

Week 52, 12 Months

Happy first birthday. Sleep may still be a bit hit and miss, and that’s genuinely common rather than a sign you’re doing something wrong. Walking arrives on a wide spectrum, anywhere from about 9 to 18 months is considered within the normal range, so don’t measure your baby against a friend’s who took their first steps at ten months.

Around Week 55: Leap 8, “Programs”

Shortly after the first birthday, your baby starts to grasp “if this happens, then that follows,” essentially the beginnings of planning and decision making. It’s a genuinely big cognitive shift, and it’s also, not coincidentally, around when a lot of babies start testing what happens if they throw food, or refuse a nappy change, or push a boundary on purpose to see your reaction. Frustrating in the moment, but it’s a sign of real thinking happening.

The Second Year: What Comes Next