My 12-week-old breastfed baby has poop t

The sour smell on its own is not a danger sign — and the switch to formula isn't the fix I'd choose; go back to breastfeeding on demand and put tonight into a 24-hour tally instead. At 12 weeks the things that decide whether this is nothing or something are colour, blood, wet nappies and feeding — not the smell.

Is the smell normal? Breastfed stool varies from day to day: colour, looseness and smell all drift, and a sour, yoghurt-like smell by itself is a common variation, not a diagnosis of anything. Smell is a poor alarm — it can't tell you the difference between "different" and "unwell". That is why the tracking below is where the value is.

1. What you already tried, and what I'd do instead

Switching to formula over the smell. Understandable, but it costs you two things. At 12 weeks your supply is still being set by how much your baby feeds. And formula stool is firmer, darker and more pungent — so the very thing you are watching gets muddier and you lose the baseline the pediatrician will ask for.

If you have already given some formula: no harm done, and no blame here. Note the brand, roughly how much per feed and how many days, then go back to the breast on demand. If you want to keep some formula in the mix, that is your call — but keep it steady for the next few days rather than switching back and forth while you are trying to see a pattern.

Better alternatives for tonight — about 20 minutes, all at home:

1. 10 min — fridge sheet. One line per feed: time, breast or bottle, how your baby was afterwards (alert, sleepy, hard to wake), wet nappy yes/no, dirty nappy colour and whether it was loose or formed, mucus or blood yes/no.
2. 5 min — photograph one dirty nappy in daylight, and write the time next to it. Keep that nappy sealed in a bag in case they want to test it.
3. 5 min — write down your baby's own normal from last week: wet nappies and feeds per 24 hours. Your own number is the one that matters, not a chart's.
4. Keep feeding on demand, day and night. Nothing added to the bottle, nothing to "thicken" it.
5. Notice the bottom: a sore red nappy area with spots around it is worth saying out loud at the visit.

2. What to avoid

• Don't give your baby anything for the smell: no probiotic or "gut" powders, gripe water, herbal drops, teas, or anything sold as a detox.
• Don't give your baby any of your own medicines — in particular not your amoxicillin. It isn't theirs, and no more of it is to be given to them.
• Don't stop breastfeeding to "rest the gut" — that works against you.
• Don't replace a feed with plain water, watered-down formula, juice or a soft drink.
• Don't keep changing formula brands while you are trying to see a pattern.
• Don't use the smell as your measure of how your baby is. Use colour, blood, wet nappies, feeding and alertness.
• No diagnosis and no medicine changes from me, and no doses for a baby.

3. Decision tree

Now — call your local emergency number (the ambulance service where you live: tell me your city and I will check the local one) or go straight to the children's emergency department if:

• blood in the stool — bright red streaks, dark red or jelly-like — or black tarry stool
• white, pale or clay-coloured stool
• green (bile-stained) vomit, vomiting that will not stop, or forceful vomiting after feeds
• feeding poorly or refusing feeds, or clearly fewer wet nappies than your baby's normal
• unusually sleepy, hard to wake, floppy, weak or high-pitched cry
• fast or laboured breathing, or blue lips
• feels hot or cold, or the temperature swings up and down
• sunken soft spot, no tears, dry mouth
• red or purple spots that do not fade when a glass is pressed on them
• a swollen or tender belly, or spells of screaming with the legs pulled up

A baby this age can be seriously unwell without a fever — don't wait for one to appear before acting.

Today or tomorrow — message or call the pediatrician (not an emergency) if:

• the smell changed and something came with it: more frequent or watery stools, mucus, a sore spotty bottom, extra fussiness, or a bit less interest in feeds
• you, the breastfeeding parent, have started an antibiotic — amoxicillin is on your own list — say so, because it is one of the ordinary things a clinician wants to know
• you gave formula for a few days and want to know whether to continue

Bring the tally sheet and the photo.

Nothing to do but watch, if your baby feeds as usual, has their normal number of wet nappies, is alert and consolable after feeds, has no blood, no vomiting and no fever, and the stool colour is their usual yellow or mustard. Then mention it at the next routine visit — the smell alone does not need an appointment tonight.

4. What to say — screenshot these

Message to the pediatrician:
"12-week-old, breastfed. Since [date] the stools smell sour, like yoghurt. Colour [ ], consistency [ ], no blood or mucus. Feeds [ ] in 24 h (usual [ ]), wet nappies [ ] in 24 h (usual [ ]), alert after feeds, no fever, no vomiting. I gave formula for [ ] days ([brand]) because of the smell and I am back to breastfeeding. I am taking [ ]. Do we need to be seen, and what would make you want to test the stool?"

On the phone: "I'm calling about my 12-week-old. The stool smell changed [how many] days ago. No blood, no fever, feeding and wet nappies are normal. I want to know whether we need to be seen, and what should bring us back."

Ask these three: Is this stool normal for a breastfed baby this age, and what would make you want to test it? Would you do a test weight and watch one feed? What exactly should bring us back, and by when?

5. What success looks like in 24 hours

Not a cure — a baseline. By this time tomorrow: the tally is filled in rather than remembered, you know your baby's own wet-nappy and feed numbers, you have one photo with the time on it, feeds are back to breast on demand, and your baby is alert and consolable after feeds with no blood and no fever. If instead the stools become more frequent or watery, blood or mucus appears, or your baby feeds less or wets fewer nappies, that is your signal to call rather than keep watching.

I have made a one-page sheet you can hand over at the visit — the tally, the change in one line, your medicine line, and the return-now list: sour-stool-12-week-baby-visit-sheet-717e0243.pdf

One thing would help me place this properly: compared with last week, are the wet nappies and the feeds the same, fewer, or more?