“Fussy Phases”: A Heads-Up on Developmental Milestones

Childrens’ developmental milestones can sneak up on the most experienced of parents. And navigating these fussy, irritable, unpredictable stages can be exhausting. Here’s some information so parents can better understand what’s going on in their child’s brain, as well as a rough timeline on when they can expect these stages to hit. 

Everything Is Fine … Until It Isn’t

If you’ve been the parent of a newborn longer than a month (and seriously, it only takes that long), you’ve had this experience … 

One day, your angelic baby is happy, your routine is set, and you’ve fallen into a false sense of security that, if you just keep doing what you’re doing, everything will be fine. 

The next day, your child has become this.


You haven’t done anything different, and your child isn’t upset for any predictable reason. They’re just angry, and you have to fix it. 

For the next week, your child is irritable, unhappy, and downright unpleasant. You question why you ever wanted to be a parent in the first place. 

And then, one day, your child wakes up looking like this again.


Again, you’ve done nothing differently, and your child’s environment hasn’t changed much to explain away their behavior. To the undiscerning eye, it seems like they simply decided they were in a bad mood and have decided they’re in a good one now.

Which is maddening. 

But if you’ve read this blog before, you’ll know that kids under 2 years old are incapable of being deceitful, stubborn, or malicious. We don’t know how to have those kinds of complex emotions when we’re that young. Especially so as infants.

So what’s happening to your child? More likely than not, the mood swing is being caused by rapid brain development.

Development Can Really Mess With Kids

Think back to the last time you updated your phone or your computer. Was it acting glitchy, irritated, and weird several days before you remembered to update your system? That’s because the programs and apps trying to run were too sophisticated for your device to keep up.

That’s essentially what happens to your baby’s brain, too.

Humans are not born with the latest “processing system.” They’re brains have to adapt and develop as they grow. And when they do, “glitchy, irritated, and weird” doesn’t even begin to describe what’s happening. Because a brain is much MUCH much more complex than a computer.

The authors of the book “Wonder Weeks” explain it this way:

Shortly before each leap, a sudden and extremely rapid change occurs within the baby. It’s a change in the nervous system, chiefly the brain, and it may be accompanied by some physical changes as well. In this book we call this a ‘big change.’ Each big change brings the baby a new kind of perception and alters the way that she perceives the world. And each time a new kind of perception swamps your baby, it also brings the means of learning a new set of skills appropriate for that world.

So when the brain is in the middle of making the “jump” into a new developmental stage, it’s a very overwhelming time for your baby. And the way babies deal with being overwhelmed is by:

  • Becoming clingy.
  • Becoming cranky.
  • Crying incessantly.

Especially for new parents, a sudden change in their child’s temperament can be scary. They may think there’s something happening with their child that they aren’t experienced enough to understand. But, if your baby is well-fed, changed, and well-rested, chances are high that they are in the middle of a developmental leap and you’re along for the ride.

When to Expect a Leap

An amazing thing about human brain development is that it is somewhat predictable. While these are definitely estimates — please don’t get anxious if your child is hitting these milestones a week or two early or late — babies tend to go through leaps and their correlated fussy periods at predictable times. They are:

  • Week 5 (Learning Sensation)
  • Week 8 (Learning Patterns)
  • Week 12 (Learning Transitions)
  • Week 15 (Learning Events)
  • Week 23 (Learning Relationships)
  • Week 34 (Learning Categories)
  • Week 42 (Learning Sequences)
  • Week 51 (Learning Programs)
  • Week 60 (Learning Principles)
  • Week 71 (Learning Systems)

Here’s the good news. The earlier leaps don’t usually last that long. (Not that three or four days of constant crying is a walk in the park. We get it.)

Here’s the not so great news. They typically happen at a more rapid pace. So while one leap may only take three days to get through, the next one may happen two weeks later.

Here’s more good news. The older your child gets, the more time you all will have to recover between each leap.

Here’s more not so great news. Some of those developmental weeks could last up to six weeks.


Here’s the ultimate good news. At the end of each of these leaps, you will get to watch your child’s personality, intellect, and temperament open up and change in ways that will amaze you. 

Offer Kindness & Compassion

The point of this article isn’t to offer a clever workaround for these difficult phases. There is no convenient “But wait!” moment to share. Children’s brains develop. The way they deal with it is by being cranky, irritable, and weepy. There is no way to fast-forward them through the process. 

But knowing that they are overwhelmed — that they are struggling to adjust to an internal process they have no control over — is an oddly comforting thing. You aren’t doing anything wrong. They aren’t in danger or in pain. They’re struggling.

And it’s much easier to have kindness, compassion, empathy, and grace for someone who is struggling than it is for someone we’ve decided is just being difficult.

So find comfort during these times of development where it feels like your child has been replaced with the absolute worst version of themselves. They’ll come out of this storm soon. The best thing you can do for them is love them through it, find ways to re-energize so you don’t lose your mind, and find joy in the new skills your child is learning once they make it to the other side.