Your Baby's Sour Smelling Stool: Is It Normal?

The stool is mushy and has a sour smell?

Hello, doctor.
I am a new mother.
After taking 42 days of maternity leave, I returned to work, and my baby is currently being cared for by my mother-in-law.
My baby is now 2 months and 15 days old, weighing 6.3 kg.
Since I returned to work, my baby has been drinking only formula milk, 150 cc at a time, six times a day.
The baby has a bowel movement every two days or sometimes twice a day.
The stool color is similar to the color of stool types 7-9, but it has a sour smell and is mushy.
My mother-in-law, who is a professional caregiver, keeps emphasizing that the sour smell is due to the formula milk, stating that it was normal for the breast milk mixed with formula to have a sour smell, but now that the baby is on pure formula, the stool should not have a sour smell.
She insists that we should change the formula.
I would like to ask the doctor if it is normal for the stool to have a sour smell.
If I need to change the formula, should I mix it gradually, one scoop at a time, or switch to the new formula directly? Thank you.

Feng Mama, 30~39 year old female. Ask Date: 2022/01/13

Dr. Zhang Yingwen reply Pediatrics

The presence of a sour smell in the stool can indeed be due to abnormal fermentation of sugars.
An excessive amount of lactose in breast milk or a high sugar content in formula, combined with the infant's underdeveloped gastrointestinal function, can impair the digestion and absorption of lactose.
Undigested lactose that enters the colon may be fermented by certain intestinal bacteria, resulting in a sour odor in the stool.
It is important to observe whether the baby experiences bloating or frequent crying; if so, the sour smell of the stool warrants attention.
On the other hand, your baby is currently 2 months and 15 days old, weighing 6.3 kilograms, which indicates good growth.
If there are no signs of poor appetite, irritability, or abdominal pain and bloating, you may continue to monitor the situation.
If you decide to change the formula, it is recommended to gradually mix in the new formula one spoonful at a time.

Reply Date: 2022/01/13

More Info

When it comes to understanding your baby's stool, especially regarding its sour smell, it's important to consider several factors that can influence the characteristics of a baby's bowel movements. In your case, your baby is 2 months and 15 days old, weighs 6.3 kg, and has transitioned to a fully formula-fed diet. The fact that your baby is having bowel movements every two days or even twice a day is generally within the normal range for infants, especially those on formula.

The sour smell of your baby's stool can be attributed to several factors. Firstly, the digestion of formula milk differs from that of breast milk. Formula is designed to be nutritionally complete, but it can lead to different stool characteristics, including odor. The sour smell could be a result of the fermentation of undigested lactose or other components in the formula, especially if your baby is sensitive to certain ingredients. It's also worth noting that the consistency of the stool being mushy is common in infants, particularly those consuming formula.

Your concern about whether the sour smell indicates a problem is valid. While some odor is normal, a significantly foul or particularly strong sour smell could indicate an intolerance or allergy to the formula. If your baby seems comfortable, is gaining weight appropriately, and does not exhibit signs of distress, the sour smell may not be a cause for concern. However, if you notice other symptoms such as excessive gas, diarrhea, vomiting, or irritability, it would be wise to consult your pediatrician.

Regarding the advice from your caregiver about changing formulas, it is essential to approach this carefully. If you decide to switch formulas, the best practice is to gradually transition to the new formula. This can be done by mixing the new formula with the old one, starting with a small amount of the new formula and gradually increasing it over several days. This method helps your baby's digestive system adjust to the new formula and can reduce the risk of gastrointestinal upset.

In summary, a sour smell in your baby's stool can be normal, especially with formula feeding, but it is essential to monitor for any additional symptoms that may indicate a problem. If you are considering changing formulas, do so gradually to allow your baby's digestive system to adapt. Always consult with your pediatrician if you have concerns about your baby's health or dietary needs, as they can provide personalized guidance based on your baby's specific situation.

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