Why Is My Baby Suddenly So Fussy?

A mother holding a baby on her lap in lamplight while the baby chews a blue silicone teether

This is a different question from why babies cry, and a much more answerable one, which is the reason I've always found it the better place to start. Here the baseline was fine until something changed. That narrows it enormously, because sudden changes in a previously settled baby come from a short list, and you can work down it in order of likelihood. The frightening explanations are at the bottom of the page, and they're at the bottom because that's where they belong on the list, a point I'd have appreciated somebody making to me at two in the morning.

1. A growth spurt

Clusters around three weeks, six weeks, three months and six months, though no baby has read the schedule.

For two or three days she feeds far more often, sleeps worse, and is impossible to please, and it looks exactly like a supply problem and isn't. The extra feeding is the mechanism that pushes supply up to meet the new demand.

Feed responsively, ride it out, expect it gone in about three days.

2. A developmental leap

New skills are disruptive while they're being built.

A baby learning to roll practises at three in the morning. One working on crawling is furious all day about being where she is. One about to talk goes clingy for a fortnight.

Fussiness that shows up a week or two before a visible new skill is so common it's almost diagnostic, though you can't see it until afterwards.

Around 4 months: sleep architecture matures permanently, and everything gets harder for a few weeks.
6 to 8 months: sitting, crawling, and the beginning of separation awareness.
8 to 10 months: pulling up, cruising, and separation anxiety proper.
12 months: walking and the one-nap transition.
Expect two to six weeks, and a return to normal afterwards.

The usual suspects

3. Teething

Real, and blamed for roughly ten times what it causes.

It disturbs things for a day or two either side of a tooth coming through, with drooling, gum-chewing and one flushed cheek.

What it doesn't cause: high fever, diarrhoea, vomiting, or a rash, all of which are illness, and calling any of them teething is how illness gets missed. Weeks of disruption blamed on a tooth is almost always something else.

4. Overtiredness that has crept in

Nap schedules drift. A twenty-minute change in a wake window three weeks ago produces a baby who's inexplicably difficult today.

A nap that has shortened, or one that has been dropped too early.
An awake window that has outgrown the baby's tolerance.
A few late nights that have accumulated into a sleep debt.
The fix is almost always an earlier bedtime, held for four nights before judging it.

5. Something in the environment changed

Take a straight look at it, because the answer is often embarrassingly simple.

A house move, a new room, a change of carer, starting nursery, a visitor staying.
A parent returning to work, or simply a different weekly rhythm.
A hotter or colder spell. Babies are sensitive to temperature shifts and it shows as fussiness before anything else.
A new sibling, which reorders everything a baby understood about their day.
Tension in the household, which babies read accurately even when nobody is saying anything.

6. Illness, coming

Fussiness usually arrives about a day ahead of anything you can see.

A baby who becomes inexplicably difficult and then produces a cold twenty-four hours later was telling you something you had no way to read yet.

The one that was never sudden

Everything above assumes something changed. Here's the cause that doesn't fit that shape, and it's the one most often missed for years.

Some babies are bothered by what they're lying on and wearing. Not allergic to it, bothered by it. A seam, a stiff label, the residue of fabric softener, a synthetic that doesn't move moisture. The baby has no way to tell you and no way to move away from it, so it comes out as tossing, waking, and a general fussiness that never quite has a reason.

It's more common than the parenting literature suggests. Ben-Sasson, Carter and Briggs-Gowan surveyed 925 children aged seven to eleven and found that 16% of parents reported at least four tactile or auditory sensations that bothered their child, roughly one in six, while the same group's follow-up work traced the pattern back into infancy, which is where it starts and where it is almost never recognised. I don't know whether the figure holds in Singapore, since nobody seems to have asked.

It gets missed because it isn't a change. It's a constant, so it reads as temperament, and a parent looking for what's different today will never find it, because nothing is different today and nothing was different last month either.

The question to ask isn't what changed. It's whether the fussiness was ever sudden, or whether you've been calling a long-running pattern a phase for a while now.

Check it with what you already own, for free. Cut the labels out. Turn a seamed sleepsuit inside out for a night. Wash the sheet again with no softener at all, since softener works by leaving a coating on the fibre, which is the opposite of what sensitive skin wants. Swap a synthetic for whatever the softest, most breathable thing in the drawer is, changing one at a time, giving each two or three nights, and seeing whether anything moves.

Two cautions. In this heat a baby who's too warm and a baby who's bothered by her sheet look identical from the doorway, so rule out temperature first, because it's quicker and more often the answer. If sensitivity to texture, sound or light is marked, don't self-diagnose it: the American Academy of Pediatrics is explicit that sensory processing disorder shouldn't be used as a standalone diagnosis, and that a child with pronounced sensory difficulty should be assessed properly, because it can go alongside things that need identifying early. Raise it with a doctor, because bedding isn't the answer to that one.

Working out which one it is

The list is long. Narrowing it isn't, if you ask the right three questions.

Did it start suddenly or build over days? Sudden points to illness or pain; gradual points to tiredness or a leap.
Is feeding affected? Feeding more suggests a growth spurt; feeding less suggests illness or pain.
Are they fussy all day or at particular times? All day points to illness; evenings point to the ordinary evening pattern.
Is anything new physically, a tooth, a skill, a rash, a temperature?
Did anything change in their week?
How long has it lasted? Two to three days suggests a growth spurt. Two to six weeks suggests a leap. Longer than that with no explanation deserves a conversation.

What helps whatever it is

You'll often never find out which of the six it was, which counts for less than it sounds, since most of the response is the same either way.

Go backwards temporarily. More contact, more feeds, an earlier bedtime, fewer outings. This isn't a habit being formed; it's meeting a temporary need.
Hold the routine steady even while offering more support. Consistency is what they orient by when everything else is unfamiliar.
Don't start something new. A leap is the worst possible moment to change approach, and it's when most people do.
Get outside. It resets babies and adults both.
Share the load. A fussy fortnight absorbs more than one person has.

When it isn't a phase

Everything above assumes a well baby having a difficult week. These don't.

A temperature of 38°C or above in a baby under three months.
A cry that is high-pitched, weak, continuous or unlike their normal one.
Floppiness, unusual drowsiness, or difficulty rousing them.
Difficulty breathing, grunting, or pauses in breathing.
A rash that doesn't fade when pressed with a glass, or a bulging soft spot.
Refusing feeds, taking under half the usual amount, or no wet nappy for 12 hours.
Vomiting that is forceful or green, or blood in the stool.
Inconsolable episodes with the legs drawn up, especially with vomiting: immediate assessment.
Weight gain that has flattened.
Any instinct that this isn't a phase. That instinct is treated as a clinical warning sign for good reason.

What I read

This guide is general information, written to help you understand a topic and ask better questions. It is not medical advice and does not replace your own doctor, midwife or paediatrician. Anything that worries you about your baby belongs with them.

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