A healthy baby may have green stools (poop) if the mother has a lot of milk and it moves through the baby quickly. The green colour comes from bile, which is released by the gallbladder. Bile mixes with the milk as it moves through the gut. When the milk moves slowly, bacteria in the gut feed on the bile and convert it into stercobilin, which makes the stool yellow. If the milk moves quickly, as it does when mothers produce a lot of milk, the bacteria don’t have time to feed on the bile, so the stool becomes green. Green stools are not a sign that your baby isn’t getting enough “hindmilk,” the high-fat milk produced near the end of a feed. A newborn babies’ first stools are green. A baby who becomes sick or has another illness affecting the bowel may also produce green stools.
A) The causes of green stools
1) The newborn transition from meconium
Newborn babies quickly change their stooling patterns. They will have a tar-like, very dark first stool (poop). This is called meconium. Subsequent stools will become lighter and green in colour and by the fifth day after birth the stools are generally yellow.
Stools that remain green after five days of age can be a sign that the mother’s milk has not come in or that the baby is underfed. Such babies should be seen by their health-care providers. These babies also stool infrequently or have very small stools and will show other signs of being underfed.
2) Quick transit of the milk through the baby's gut
Green stools can also appear later in the first month when a mother has a large milk supply and it moves quickly through the baby’s body.
One study (Gustin 2018) of normal babies showed that about 60% had yellow stools, 30% of the babies studied had green stools, and the remainder had either brown or orange stools.
To understand how this happens, it helps to know a bit of body chemistry:
Red blood cells circulate in the blood. They contain an iron-containing compound called hemoglobin that helps to move oxygen through the body and deliver it to cells so that they can function. Hemoglobin gives blood its red colour.
Once red blood cells are older than three months, the body destroys them.
The hemoglobin is broken down into a compound called biliverdin which is green in colour. This is added to other substances including water and fats by the liver, to create bile. Bile is dark green to yellow brown in colour.
Bile leaves the liver through ducts (tubes) and is stored in the gallbladder.
When the baby takes in milk, the gallbladder squirts some bile into the bowel through the bile duct.
The bile then travels through the gut (intestines) along with the milk. Bile helps the baby absorb fat.
A lot of bacteria live in the gut. These bacteria digest biliverdin for energy and in the process create stercobilin, which is yellow. Stercobilin gives a baby’s stool its yellow colour and adult stool its brown colour.
If the bile-milk mixture passes quickly through the baby, which happens when the baby drinks a lot of breast milk as when the mother has a large milk supply, the bacteria don’t have time to grab the bilirubin on its way by, so the bile and stool stay green.
3) Abnormal causes of green stools
Green stools can also appear when the baby is sick with diarrhea or another illness affecting the bowel. If your baby is suddenly unwell and has diarrhea, please see your health-care providers.
B) Misinformation about green stools
Some mothers are told that their baby’s stools are green because their baby is not getting enoughhindmilk, the high-fat milk produced toward the end of a feeding. There is no relationship between milk fat content and stool colour. Bile, not the presence or absence of milk fat, produces the green colour. There is no need to change the breastfeeding routine or to force the baby to stay on one breast.
Gustin J, Gibb R, Kenneally D, et al. Characterizing Exclusively Breastfed Infant Stool via a Novel Infant Stool Scale. JPEN J Parenter Enteral Nutr. 2018 Nov;42 Suppl 1:S5-S11