There is no single reason babies develop colic, and that’s part of what makes it so frustrating for parents. Colic affects 10 to 30 percent of infants worldwide, hits boys and girls equally, and almost always resolves on its own by 3 to 4 months of age. The best current evidence points to several overlapping factors: an immature nervous system that can’t filter out stimulation, imbalances in gut bacteria, hormonal signals that speed up digestion uncomfortably, and in some cases, sensitivity to proteins in milk.
What Colic Looks Like and How Long It Lasts
Colic typically starts around 3 weeks of age and worsens between 4 and 6 weeks. Most colicky babies begin improving after 6 weeks and cry less than an hour a day by 12 weeks. The classic pattern is intense, inconsolable crying that tends to cluster in the late afternoon or evening, often with a red face, clenched fists, and drawn-up legs. There’s nothing obviously wrong with the baby, and feeding, changing, and holding don’t reliably stop the crying. That’s the hallmark distinction: the crying has no clear external cause and resists the usual soothing strategies.
An Immature Nervous System
One of the most widely accepted explanations is that colicky babies are unusually sensitive to stimulation and haven’t yet developed the ability to regulate their own nervous system. After spending months in the muffled, constant environment of the womb, a newborn is suddenly flooded with light, sound, touch, and movement. Most babies can gradually tune out excess input, but some can’t. They absorb everything, become overwhelmed, and the only outlet they have is crying.
This fits neatly with the timeline. Colic peaks during the period when babies are most alert but least able to self-soothe, then fades as the nervous system matures and the infant develops better coping mechanisms, like looking away from a stimulus or sucking on their hand.
Gut Bacteria and Inflammation
A growing body of evidence ties colic to what’s happening inside the gut. Research published in The Journal of Pediatrics found that babies with colic have measurably different bacterial populations in their intestines compared to non-colicky babies. Specifically, colicky infants had fewer Bifidobacteria, a group of beneficial bacteria that dominate the healthy infant gut and help regulate inflammation. They also had higher levels of certain bacteria associated with gut irritation.
This bacterial imbalance, called dysbiosis, may produce low-grade inflammation in the intestinal lining. It’s not an infection or a disease, but it could create enough discomfort to trigger prolonged crying in a baby who can’t tell you their stomach hurts. Whether this imbalance is a cause of colic or a consequence of it isn’t fully clear, but the association is consistent across multiple studies.
Hormones That Speed Up Digestion
Two hormones appear to behave differently in colicky babies. The first is motilin, which controls how quickly the stomach empties and how fast food moves through the intestines. Colicky infants have elevated baseline levels of motilin, which may push food through the gut too quickly, causing cramping and discomfort.
The second is serotonin, a chemical most people associate with mood but which also plays a major role in gut function. Colicky babies show higher levels of a serotonin byproduct in their urine, suggesting their intestines are producing more serotonin than usual. Since serotonin drives intestinal contractions, excess amounts could contribute to the kind of abdominal pain that makes a baby pull their knees to their chest and scream.
Milk Protein Sensitivity and Lactose
In a subset of colicky babies, the problem traces back to what they’re eating. Cow’s milk protein allergy is the most studied dietary trigger. When a baby’s immune system reacts to proteins in cow’s milk (whether from formula or passed through breast milk), it can cause stomach pain, gas, diarrhea, and fussiness that looks identical to colic. Skin reactions like eczema or a rash sometimes accompany the digestive symptoms, which can help distinguish an allergy from plain colic.
Temporary lactose overload is a separate issue. This isn’t the same as lifelong lactose intolerance. It happens when a young baby’s gut receives more lactose than it can break down at once, producing excess gas and bloating. Babies can also develop temporary lactose intolerance after a stomach bug. The symptoms overlap heavily with colic (gas, fussiness, belly pain), which is why some parents find it hard to tell whether they’re dealing with colic, an allergy, or a feeding issue.
A Possible Link to Migraine
One of the more surprising findings in colic research is its connection to migraine. A systematic review pooling data from multiple studies found that 66% of children who later developed migraines had been colicky as infants, compared to just 23% of children without migraines. That translates to roughly a sixfold increase in odds.
Researchers have speculated that colic may actually be a migraine-like response to overstimulation. The timing supports this theory: colic resolves around 3 months, which is when babies begin producing melatonin and developing day-night sleep rhythms. Melatonin is known to play a role in migraine regulation, so its arrival could explain why the crying finally stops. This doesn’t mean every colicky baby will get migraines later, but it suggests that at least some cases of colic involve the same neurological pathways.
Smoking During Pregnancy
Prenatal nicotine exposure is one of the few environmental factors consistently linked to colic risk. The American College of Obstetricians and Gynecologists lists infantile colic among the conditions more common in babies born to mothers who smoked during pregnancy, alongside respiratory infections, asthma, and childhood obesity. The mechanism isn’t fully pinned down, but nicotine affects both nervous system development and gut motility, two systems already implicated in colic.
Why It Resolves on Its Own
The fact that colic almost universally disappears by 3 to 4 months tells us something important: whatever is causing it is tied to the developmental stage of early infancy. The nervous system matures and learns to filter stimulation. The gut microbiome stabilizes and diversifies. Hormonal signaling in the digestive tract evens out. Melatonin production kicks in and helps organize sleep cycles. All of these changes converge in the same narrow window, which is why colic rarely lingers.
One placebo-controlled trial of a probiotic for colic found that 66% of babies given a placebo improved within three weeks, reinforcing what pediatricians have long observed: most colicky babies get better regardless of intervention, simply because they’re growing out of it. That doesn’t make the weeks of inconsolable crying any easier to endure, but it does mean the condition is self-limiting in the vast majority of cases.