There is no single, definitive cause of colic. Instead, it appears to result from a combination of factors, including an immature digestive system, disruptions in gut bacteria, sensory overload, and in some cases, food sensitivities. Colic affects 10% to 30% of infants worldwide, typically starting in the first six weeks of life and resolving on its own by three to four months, sometimes lasting up to six months. The classic definition, known as the “rule of three,” describes a healthy, well-fed infant who cries for more than three hours a day, more than three days a week, for more than three weeks.
Despite decades of research, no one has found a single explanation that accounts for every colicky baby. What researchers have found are several overlapping contributors, each of which plays a role for some infants and not others.
Gut Bacteria Imbalances
One of the strongest lines of evidence points to the bacteria living in a baby’s digestive tract. A study published in The Journal of Pediatrics found that infants with colic had measurably different gut bacteria compared to non-colicky infants. Specifically, colicky babies had significantly fewer Bifidobacteria, a group of beneficial bacteria that help with digestion and immune function. They also had higher levels of certain bacterial species associated with inflammation.
The study also found elevated levels of a protein called calprotectin in the stool of colicky infants. Calprotectin is a reliable marker of gut inflammation. This finding held true regardless of whether the baby was breastfed or formula-fed, suggesting that something about the bacterial environment itself, not just diet, contributes to the discomfort. Essentially, a colicky baby’s gut may be inflamed and poorly colonized with the “right” bacteria, leading to gas, cramping, and distress.
Cow’s Milk Protein Sensitivity
For a subset of colicky babies, the culprit is an immune reaction to proteins in cow’s milk. Cow’s milk protein is the leading cause of food allergy in children under three. While 5% to 15% of infants show symptoms that suggest a reaction to cow’s milk protein, confirmed allergy affects roughly 2% to 7.5% of young children. Even exclusively breastfed babies can be affected, though at a lower rate of about 0.4% to 0.5%, because milk proteins from the mother’s diet pass through breast milk.
Symptoms of cow’s milk protein sensitivity can look identical to colic: prolonged crying, gassiness, fussiness during or after feeds, and sometimes loose stools or skin rashes. Soy, eggs, and wheat are other potential triggers, though cow’s milk is by far the most common. When a food sensitivity is the underlying issue, removing the offending protein from the breastfeeding mother’s diet or switching to a specialized formula often leads to noticeable improvement within a few days to two weeks.
An Immature Digestive System
A newborn’s gut is still learning how to process food efficiently. The muscles that move food through the intestines contract in uncoordinated patterns during the first months of life, which can trap gas and cause cramping. Some infants may also have temporarily low levels of lactase, the enzyme that breaks down the sugar in milk. When lactose isn’t fully digested, bacteria in the large intestine ferment it, producing gas, bloating, and discomfort. This is different from true congenital lactase deficiency, which is extremely rare and causes severe diarrhea and weight loss. In most colicky babies, any shortfall in lactase production is mild and temporary, resolving as the gut matures.
Acid Reflux and GERD
Many parents wonder whether their baby’s crying is caused by reflux. Most babies spit up, and simple reflux in an otherwise happy, growing infant is not a concern. Gastroesophageal reflux disease (GERD) is different. A baby with GERD is visibly uncomfortable: arching the back during or after feeding, refusing to eat, vomiting forcefully six or more times a day, gagging or choking frequently, and failing to gain weight.
GERD can produce symptoms that look exactly like colic, including prolonged crying and irritability. But GERD is not the only reason for colic, and many colicky babies have no reflux at all. The overlap in symptoms makes it genuinely difficult to distinguish between the two. Signs that reflux may be involved include the timing of distress (during or right after feeds rather than in the evening), frequent forceful spit-up, and poor weight gain. If those features are absent, reflux is less likely to be the explanation.
Nervous System Overload
A baby’s nervous system is brand new. In the first weeks of life, it is still learning how to regulate sleep, process sensory input, and coordinate digestion. Some researchers believe colic reflects a temporary inability to filter out stimulation. After a full day of sights, sounds, and touch, a baby’s system becomes overwhelmed and the only outlet is prolonged, inconsolable crying.
This theory helps explain one of colic’s most distinctive patterns: the crying tends to cluster in the late afternoon and evening, after hours of accumulated stimulation. It also explains why colic resolves around three to four months, which is roughly when infants develop more mature self-soothing abilities and better sensory regulation. The vagus nerve, which connects the brain to the gut and plays a central role in calming the body down, is still maturing during this period. When it isn’t functioning optimally, digestion can slow and the baby may become uncomfortable more easily.
Maternal Smoking
Exposure to tobacco smoke is one of the most clearly documented environmental risk factors for colic. Research published in Archives of Disease in Childhood found that the prevalence of colic was roughly twice as high among infants whose mothers smoked compared to infants of non-smoking mothers. The risk increased with the number of cigarettes: mothers who smoked 15 or more cigarettes per day had nearly three times the odds of having a colicky infant compared to non-smokers.
Interestingly, breastfeeding appeared to partially buffer the effect. Infants of smoking mothers who were formula-fed had a higher risk of colic than those who were at least partially breastfed. The exact mechanism is unclear, but nicotine and other tobacco compounds may affect gut motility or alter the developing nervous system in ways that increase fussiness.
What Helps: The Role of Probiotics
Because gut bacteria play such a prominent role, probiotics have been one of the most studied interventions for colic. A pooled analysis of four clinical trials found that a specific probiotic strain reduced crying and fussing time by about 46 minutes per day at three weeks in breastfed infants. That is a meaningful reduction when a baby is crying for hours on end. However, the same benefit did not appear in formula-fed infants, possibly because the gut environment differs between the two feeding methods.
This difference underscores that colic likely has more than one pathway. A baby whose colic stems primarily from bacterial imbalance may respond to probiotics, while a baby reacting to cow’s milk protein needs a dietary change, and a baby overwhelmed by sensory input may benefit most from reduced stimulation and consistent soothing routines. In many cases, more than one factor is at play simultaneously.
Why It Resolves on Its Own
The single most reassuring fact about colic is its timeline. It peaks around six weeks and resolves by three months in most infants, with nearly all cases gone by six months. This pattern lines up with the maturation of the gut, the diversification of intestinal bacteria, and the development of the nervous system. As a baby’s digestive enzymes reach full production, gut bacteria stabilize, and sensory regulation improves, the conditions that fuel colic gradually disappear. The crying doesn’t stop because parents found the perfect fix. It stops because the baby’s body caught up.