Why baby's pee does not smell, but baby's poo does

You notice it in the first week. A wet nappy smells of almost nothing — warm cotton, maybe. The next one clears the room. Same baby, same day, same milk.

Urine leaves the body sterile

Urine is, in a healthy baby, essentially sterile when it leaves the bladder. What gives older urine its sharp smell is ammonia, and ammonia is not in urine to begin with. Urine carries urea. Ammonia appears later, when bacteria on the skin and in the nappy release an enzyme called urease that splits urea into ammonia and carbon dioxide.

That takes time. A nappy changed promptly never gets there. A nappy left overnight, or a bucket of soaking cloth nappies, absolutely does — that eye-watering morning smell is the same chemistry, given hours instead of minutes.

There is a second reason newborn urine is mild: babies cannot concentrate urine well. The kidneys are still maturing, and a milk-only diet is mostly water. Dilute urine means less urea per drop, and less to work with.

Stool arrives already colonised

Stool is the opposite case. It is not sterile and was never meant to be — by weight, a large share of it is bacteria. Those bacteria have been fermenting in the gut for hours, and fermentation produces volatile compounds: hydrogen sulphide, skatole, indole, short-chain fatty acids. Volatile is the operative word. They evaporate at body temperature and travel.

So the smell does not develop in the nappy. It arrives fully formed.

Why breastfed nappies are the gentle exception

Most parents of a breastfed newborn will say the poo smells almost sweet, or faintly of yoghurt. That is real. A breastfed infant gut is dominated by Bifidobacteria, fed by the oligosaccharides in human milk that the baby cannot digest but the bacteria can. Those species mainly produce lactic and acetic acid — sour, not foul. The sulphur-containing compounds that make adult stool smell adult come from protein breakdown, and there is very little protein reaching the colon on a milk diet.

Then solids start. The gut community diversifies within weeks, protein and sulphur-containing amino acids arrive in the colon, and the nappies change character permanently. Nobody warns you about that particular milestone.

The practical corollaries

• A strong ammonia smell in a fresh nappy is worth noticing. It usually means the nappy sat too long, or, with cloth, that detergent residue is holding bacteria. It can also mean concentrated urine from a baby who is not getting enough fluid.
• Ammonia burns are chemical, not infectious. The fix is faster changes, a proper strip-wash for cloth, and time with the nappy off.
• A sudden, sharply different stool smell — alongside a change in colour, frequency or the baby's mood — is the one to mention to your consultatiebureau or GP. The baseline smell is not a problem. A change from it is information.

This post is for information only and is not medical advice.