In newborns, one of the most common causes of blood in the stool is swallowed maternal blood. This occurs either during the delivery process or during breastfeeding if the mother’s nipples are cracked and bleeding. If a maternal source is considered, then one can perform the Apt test to determine the origin of the blood. In this test, stool is mixed with water and centrifuged. Sodium hydroxide is added to the supernatant. If the blood is from the mother, the adult hemoglobin will denature and the sample will turn light brown. However, if the source of the blood is the infant, the sample remains pink because fetal hemoglobin is resistant to denaturation.

Anorectal fissures are another very common cause of rectal bleeding in children younger than 12 months. These fissures are usually seen upon close examination of the perineal skin and the anal canal. Often, they are small and caused by the passage of hard stool and resolve once the underlying constipation has been addressed.

Another common cause for blood in an infant’s stool is food allergies. The top common allergens are cow’s mild products and soy. Milk allergies occur in approximately 2% to 3% of infants, and it is typically outgrown by the age of 3 to 5 years. Symptoms include loose stools, vomiting, gagging, irritability or colic, and skin rashes. Avoiding milk-containing products is the treatment, and since January 2006, all U.S. food makers must clearly state on the package labels whether the foods contain milk or milk-based products. For infants, using a soy-based formula will sometimes work, but some infants may require using a hypoallergenic formula that contains pre-digested proteins.