Standing in the baby aisle looking at a wall of bottle teats is a strange kind of pressure. Slow flow, medium flow, size 1, size 2, size 3, it can feel like information overload. Bottle teat sizes really come down to one thing, which is how quickly the milk comes out. Get that right and feeds tend to feel calmer for everyone. Here is what the sizes actually mean, how to tell when your baby needs a change, and why plenty of babies never need to size up at all.
Please note: This article provides general information only and should not replace medical advice. If you're concerned about your baby's diet or health, always speak to your GP. Always seek professional support.
What do teat sizes mean?
A teat size tells you how fast milk flows out of the bottle. That is it. A smaller hole, or a single hole, gives a slower flow. A bigger hole, or several holes, gives a faster one. The number on the packet is a flow rate, nothing more.
Two things it does not mean. It is not about the size of your baby's mouth, and it is not about how much milk they drink. A big baby with a big appetite can be perfectly happy on a slow flow teat. It depends on the baby, and on how they like to feed.
It is also worth knowing that the NHS says there is no evidence one type of teat or bottle is better than any other. So, if you are worrying that you bought the wrong brand, you almost certainly did not.
Flow is not only about the teat, either. The NHS advises keeping the bottle horizontal and only just tipped, which lets the milk flow steadily and helps stop your baby taking in air. Combined with responsive, paced feeding, that alone can fix a feed that feels too fast or too slow, without changing a thing about the teat.
The different teat sizes
Here is the bit nobody tells you. Teat numbering is not standardised. A size 2 on one brand of bottle is not the same flow as a size 2 on another. There is no industry rule, so every brand sets its own scale and its own suggested ages. If you switch bottle brands, you cannot simply carry the number across. Always read the pack.
The general shape is the same almost everywhere, though. You start with a newborn or slow flow teat, and the options get faster from there. Two real examples show how differently brands label the same idea.
MAM runs five sizes. Size 0 is extra small for newborns, size 1 is from birth, size 2 is from around two months, size 3 is from around four months, and size X is from six months and designed for thicker liquids.
Dr Brown's uses levels instead. There is a preemie teat, then Level 1 from birth, Level 2 from around three months, Level 3 from around six months, Level 4 from around nine months, and a Y-cut teat from nine months.
Same principle, different labels, and different suggested ages for roughly the same step up. Alongside the standard range you will also see a few specialist options:
- Extra slow or preemie teats, made for premature babies and for babies who like to take their time.
- Variable flow teats, where the flow changes depending on how the bottle is turned.
- Y-cut teats, which are made for thickened milk. Only use these if a health professional has advised thickening your baby's feeds. It is never a decision to make on your own.
Whatever you use, teats wear out. Check yours before every feed and replace it as soon as you spot thinning, tears or weak points, which matters more once your baby has teeth. And do sterilise bottles and teats until your baby is at least 12 months old, as the NHS recommends. If you are still building your bottle feeding kit, keep spare teats in more than one flow rate.
When to size up
Size up when your baby shows you the flow is too slow, not when they reach a certain age. You’ll notice certain changes in their behaviour when feeding, and once you know them, they are hard to miss.
- Feeds are taking noticeably longer than they used to.
- Your baby gets frustrated partway through, fussing or crying at the bottle.
- They pull off the teat, then want it straight back.
- They chew or bite the teat rather than sucking on it.
- They lose interest halfway, or fall asleep before finishing.
Before you buy anything, check the teat itself. If it goes flat while you are feeding, the NHS suggests pulling gently on the corner of your baby's mouth to release the suction. If it looks blocked, swap it for another sterile teat. Sometimes the problem is a tired or blocked teat rather than the wrong size altogether.
Now the important part. Not every baby needs to size up. Some are perfectly content on a slow or medium flow for their whole bottle-feeding journey. The ages printed on the packaging are a rough guide, not a must-follow. Babies who also breastfeed often prefer a slower flow, because it is closer to the pace they are used to at the breast.
If you do want to try the next size, change one size at a time, test it on a relaxed feed rather than when baby is hungry or tired, and keep the old teats in case you want them back. And if feeds already feel calm, your baby is gaining weight steadily and they’re having plenty of wet and dirty nappies, you can leave things exactly as they are. Following your baby's feeding cues will tell you far more than a chart will.
How to tell if your flow is too fast
A teat can absolutely flow too fast, and this gets talked about far less than sizing up. The signs are usually obvious:
- Gulping, or swallowing hard between mouthfuls.
- Coughing, spluttering or choking on the milk.
- Milk spilling out of the corners of their mouth.
- Pulling away or arching back in the middle of a feed.
- Getting upset during the feed rather than after it.
- Bringing milk back up once the feed is finished.
The NHS is clear on this one: check that the hole in your baby's teat is not too big, because drinking milk too quickly can make your baby sick. It is a small check and an easy fix.
There is a knock-on effect too. Feeding very fast often means swallowing more air along with the milk, which can leave a baby with trapped wind, hiccups and a very unsettled hour afterwards. Some parents believe colic is the reason behind their baby’s discomfort, when the real culprit is a teat flowing faster than their baby can handle. To be clear, a fast teat does not cause colic, and colic has no single known cause. But it can add to the discomfort, and it is one of the easier things to rule out.
If you think the flow is too fast, work through this in order:
- Drop back down a size.
- Keep the bottle horizontal rather than tipped up.
- Hold your baby semi-upright with their head supported so they can breathe and swallow comfortably.
- Build in pauses, and wind them partway through rather than only at the end.
One safety point too: never leave your baby alone to feed with a propped-up bottle, and never leave them in bed with one, both put baby at risk of choking.
If sickness happens a lot, if your baby is violently sick, if they seem to be in pain, or if you are simply worried, speak to your health visitor or GP.
The takeaway
Bottle teat sizes are a flow rate, not a milestone. Keep these three things in mind. Sizes control how fast the milk comes out, and nothing else. Your baby sets the pace, not the calendar or the number on the box. And the change can go down as easily as it goes up, which is the part most guides forget to mention.
Watching your baby closely over a couple of feeds usually tells you what you need to know. If it still does not feel right, reach out to your health visitor for further support.
For more on getting bottle feeds settled, the Kendamil guide to how to bottle-feed walks through positioning, pacing and winding step by step. And if you are still putting your kit together, the Kendamil list of essential bottle feeding items covers what you actually need, and what you can safely skip.
Important Notice
Breastfeeding gives babies the best start, but we understand that every family’s journey is different. Follow-On Milk is suitable for little ones over 6 months old, as part of a varied diet. Chat with your healthcare professional for more guidance.
