In daily conversation, when we discuss/say about the baby excrement/faeces.
Which word should we used? Excrement, faeces or poop?
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In daily conversation you would say poop or poo in American English.
Excrement is used when talking about feces (the AmE spelling) being disgusting and filthy.
Feces is usually used to describe it in a more clinical or studied way, as to a doctor or scientist.
We also say that a baby has a dirty/soiled diaper. You could also simply say that a baby is dirty, or that the baby needs changing.
You might also hear someone say that a baby went number two. Number one would be peeing. This is very informal.
For wild animals, the word scat is often used.
For farm animals, the word manure is used.
Any of these might jokingly be used to describe a baby's poop.