The 2 Most Common Medical Causes of Colic in Infants

This article is an excerpt from the Shortform book guide to "The Happiest Baby on the Block" by Harvey Karp. Shortform has the world's best summaries and analyses of books you should be reading.

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What are the medical causes of colic in infants? Does colic indicate a worse problem?

Harvey Karp argues a majority of colic causes are due to over- or understimulation. However, his book The Happiest Baby on the Block acknowledges there are some instances where underlying medical issues are responsible.

Let’s look at the medical and health-related causes of colic and how to address them.

How Underlying Medical Conditions Cause Colic

Karp explains that while many people and even some doctors assume all colic has a medical cause, only 5-10% of colic cases have an underlying medical cause. In these cases, your baby’s cries are signs of discomfort due to physical symptoms. Karp notes the most common physical causes of colic in infants: allergies and constipation. Let’s explore how to manage each in more detail. 


Allergies are responsible for 90% of medically-caused colic. Babies can be exposed to common allergens—such as cow’s milk, eggs, peanuts, tree nuts, wheat, soy, and fish—through breast milk depending on the breastfeeding parent’s diet. 

Karp explains that babies with allergies tend to cry throughout the day and night. They may also have diarrhea, indicated by mucus in their stools. If you suspect your breastfed baby has an allergy, start keeping track of how long and how often they cry. Then, have the breastfeeding parent cut the foods listed above out of their diet for a week. If your baby’s crying stays the same, they likely don’t have an allergy. If your baby starts crying less, reintroduce small amounts of these foods to the parent’s diet one at a time to see if one makes your baby cry more—if one does, it’s likely their allergen.

(Shortform note: If you’re hoping to avoid your child developing allergies altogether, breastfeeding may be the best preventive measure—research suggests breastfed babies tend to suffer fewer food allergies than their formula-fed counterparts. While scientists aren’t exactly sure why, some believe it has to do with certain molecules and microorganisms unique to breastmilk that provide genetic instructions to help protect babies from developing food allergies.)


Karp explains that some cases of colic can be due to pain or discomfort from constipation. If your baby is bottle-fed, you can try improving their bowel movements by changing their formula mix or adding a tablespoon of water or prune juice to their formula once per day. Otherwise, you can try helping your baby coordinate their rectum by bicycling their legs, massaging their bottom, or inserting a greased thermometer about a half inch to an inch into their anus.

Signs of Constipation

Recognizing when your baby is constipated can be difficult, as babies naturally have fewer bowel movements than adults. But there are several signs pediatricians point to as indicators of constipation:

Your baby passes harder, drier, or more pellet-like stools.
Your baby seems to struggle with passing stools, arching their back or crying while doing so.
Your baby’s general rhythm of bowel movements changes.
If you notice your baby showing any of the above symptoms, consider trying some of the above methods and seeing if they help.

When to See the Doctor

In addition to the more common medical causes of colic, Karp also notes several signs that indicate you should take your baby to the doctor. Note that while these signs may indicate more serious issues, said issues are responsible for less than one percent of medically-caused colic. Karp recommends seeing a doctor if:

  • Your baby pees fewer than five times per day and/or has dark yellow urine.
  • Your baby gains fewer than four to seven ounces of weight per week (about a pound every two weeks).
  • Your baby cries all day long without any periods of calm, or has a particularly shrill, high-pitched, sharp cry.
  • Your baby is lethargic—they sleep twice as long as usual or have a weak suck.
  • Your baby cries during or right after feeding.
  • Your baby’s temperature goes below 97.5° or above 100.4°.
  • Your baby vomits more than an ounce at a time, more than five times a day, or has yellow or green vomit.

(Shortform note: While some of the symptoms listed above are easy to objectively recognize as serious—a high fever, frequent vomiting, or lack of weight gain, for example—others (like whether your baby is lethargic) are a bit more based on subjective observation. Ideally, you should have an easy-to-contact, reliable pediatrician to ask about both “objective” and “subjective” symptoms. Parenting experts note that a good pediatrician should not only recognize problems and provide treatment, but should also be available to answer your questions and provide reassurance. And whether or not your baby requires medical treatment for their symptoms, you’ll likely feel better for having asked.)

The 2 Most Common Medical Causes of Colic in Infants

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Here's what you'll find in our full The Happiest Baby on the Block summary:

  • A helpful guide for parents who have babies with colic
  • A theory about why babies develop colic, and how to manage it
  • How to improve your baby's sleep cycles (and thus your own)

Katie Doll

Somehow, Katie was able to pull off her childhood dream of creating a career around books after graduating with a degree in English and a concentration in Creative Writing. Her preferred genre of books has changed drastically over the years, from fantasy/dystopian young-adult to moving novels and non-fiction books on the human experience. Katie especially enjoys reading and writing about all things television, good and bad.

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