Infant digestion guide: what to expect

Every new parent worries about their baby, especially when it comes to their digestion and how they’re feeding. From the wonderful world of baby poo and reflux, burping and colic, it can be hard to tell what’s ‘normal’ as your baby’s digestive system develops, and what could be a sign of digestive issues.

The fact is, your baby’s digestive system isn’t fully formed when they’re first born, so what’s normal for them will be different to what’s normal for an older baby. Lots of the signs you might take as symptoms of infant digestive problems are actually completely normal¹.

While your baby might not come with a manual, we’re here to help you understand all the ins and outs of your baby’s digestive system as it develops, milestones to look out for, and what you can expect along the way. We’ll also cover practical ways you can support your little one’s infant digestive system and when it’s worth talking to your GP for reassurance and advice.

When your little one arrives into the world, they still have lots of developing to do – and that includes their digestive system. Let’s take a look at what’s happening and why.

When your baby is born, their stomach is tiny. In fact, it can only hold about a teaspoon (5-7ml) of milk at a time, which is why they need feeding little and often². This doesn’t mean they’re not getting enough milk, or that they’re not satisfied after a feed – they just can’t hold more than a tiny bit of milk at a time!

Over the first few weeks, your baby’s stomach will grow to the size of a walnut (day 3), an apricot (1 week) and then an egg (1 month), and you’ll start to see feeds gradually space out².

When your baby is very young, the muscle in their oesophagus (or food pipe) that stops milk coming back up can be weak³. That’s why they might bring up milk during or not long after feeds (which is called reflux). As they get older, this muscle will get stronger, and the reflux should get better on its own by the time they’re a year old.

When they’re first born, your baby’s body doesn’t make the same enzymes that adults and older children have to help break down nutrients like fats⁴. Breast milk contains enzymes that help support your baby’s digestion, and infant first formulas are formulated to meet the nutritional needs of infants as their digestive system develops⁵. As they get older, your baby will start making the enzymes they need on their own to support their digestion.

There’s some evidence that very young babies struggle to pass wind or poo, which can cause stomach discomfort in newborns⁶. They might grunt, squirm, turn red, or pull their legs to their chest because they haven’t yet learned how to relax their muscles in the right way – this is completely normal and doesn’t necessarily mean there’s anything wrong^{1, 7}. It usually improves by the time your baby is 3 months old.

Your gut microbiome is made up of trillions of microbes that support your digestive system⁸. As well as breaking down food and absorbing nutrients, your gut microbiome supports your immune system, helps to control blood sugar, and plays a role in managing your mood⁸.

Your baby’s microbiome is very immature at birth, which can make the infant digestive system more sensitive. It develops rapidly in your baby’s first few months of life, goes through a major transition once they start eating solid foods, and continues to develop as your little one becomes a toddler⁹. 

Laying the foundations for a healthy gut in infancy can support long-term gut health.

As your baby’s digestive system develops, it can feel tricky to keep track of what to expect. Use our handy table below for a quick view of how and when your newborn’s digestive system matures, and read the summaries below for a deeper dive into each stage.

Your baby’s digestive system is going through big changes as they get used to feeding and life outside the womb. Their tiny stomach will grow from the size of a cherry to the size of an egg, and they’ll still likely need to be fed little and often (at least every couple of hours². 

When they’re born, your baby’s first poo (called meconium) will be dark, sticky, and a green or black colour. As they get used to milk, breastfed babies’ poos will be looser and look a little like mustard seeds. Formula-fed babies’ poos are usually firmer, and they might not poo as often¹².

You might also notice that your baby spits up milk, grunts or wriggles after feeds. This doesn’t necessarily mean your baby’s in discomfort after feeding – their digestive system is just still changing and developing as they grow^{3, 1} .

Just as you and your baby are getting into a rhythm, it’s normal for some infant digestive problems to crop up at 1-3 months. Colic, gas and reflux can peak around this time (usually around 6-8 weeks), and you might feel worried that something’s wrong^{10, 3}. It might not feel like it when you’re in it, but these phases are very normal and will pass with time – most babies with colic get better by around 3-4 months.

Every baby is different, but as a brief baby poo guide, formula-fed babies will usually poo once a day at this stage. When they’re older than 6 weeks, breastfed babies can go for up to 10 days without having a poo – as long as their poo isn’t hard or dry this doesn’t mean they’re constipated^{1, 12}.

Around now, your baby’s digestive system is getting ready for solid foods. While their stomach continues to grow and their gut develops, they also start to lose their ‘tongue thrust’ reflex¹³. When your baby is younger, this reflex helps prevent choking by pushing things out of their mouth. At around 6 months, this reflex tends to disappear so they’re ready to try solid foods.

To account for this infant digestive system development, the NHS recommends that you start weaning your baby (introducing solid foods) at around 6 months, and not before 4 months¹⁴.

This is an exciting stage in your baby’s development – they’re ready to start trying solid foods!¹¹ Their body is now producing enzymes to help them break down a wider variety of nutrients as they work their way through new tastes and textures⁴.

As their diet changes, the contents of your baby’s nappy will change too. Their poos will get darker, thicker and smellier, and you might notice some bits of undigested food in there too⁷.

As your baby approaches their first birthday, they’ll likely be eating a wide variety of yummy foods, and they’ll rely less on milk as their main source of nutrition. This means their gut microbiome is getting more diverse, too⁸. Although their digestive system won’t be fully mature for another couple of years, it’s well on its way!

When you bring your baby home, it’s normal to worry about every grunt and every hiccup – you just want them to be happy and healthy! The good news is that most of the time, these things are completely normal and not a cause for concern.

Here are some of the most common infant digestive system behaviours we know new parents worry about (and what they actually mean).

Spitting up (or posseting) is when your baby brings a little milk back up after their feed. This usually happens because the muscle in the back of their food pipe is still developing and doesn’t yet stop milk coming back up from their stomach¹⁵. 

As long as your baby is feeding well and gaining weight, posseting is usually nothing to worry about. In fact, it affects up to 40% of babies¹⁶. As your baby develops, the muscle in their food pipe will get stronger, and they’ll gradually stop on their own¹⁵. Until then, it’s a good idea to keep a muslin cloth handy to clean up any spit-up.

You might think grunting, straining, or turning red is a sign of stomach discomfort in infants. But, usually, it’s just your baby’s way of learning a new skill. They’re still working out how to relax the muscles in their pelvic floor to pass wind or have a poo (it’s not as easy as you’d think!)⁷.

Most of the time, your baby’s not in pain or constipated – they’re just putting in a little extra effort to get things moving.

Your baby might have colic if they cry for more than 3 hours a day, 3 days a week for at least 1 week with no clear cause, and they’re otherwise healthy¹⁰. Having a colicky baby can feel really tough – they seem so upset, and there’s little you can do to comfort them. But we’re here to assure you that you haven’t done anything wrong, and you will get to the other side!

While professionals still aren’t 100% sure what causes colic, many agree that it’s related to babies’ developing digestive systems. It can start when your baby’s just a few weeks old, often peaking at around 6 weeks, and often clears up on its own by the time they’re 3 or 4 months old¹⁰.

If you’re worried about your baby, you can always speak to your GP or call 111 for reassurance and advice.

Just as no two adults’ toilet habits are the same, so is the case with baby poo! While one baby might poo every day, another might go several days without having a poo (especially if they’re breastfed). Baby poo also varies a lot in how it looks depending on how you feed them – it could look more yellow or green, for example¹.

It’s a good idea to keep an eye on what your baby’s poo usually looks like and note any changes, but remember that the contents of their nappy will look different as they get older. As long as they’re feeding and growing and seem happy, there’s probably no cause for concern¹.  

It’s normal for your little one to hiccup, especially during feeds. It might be a symptom of reflux or simply that they’ve swallowed air while feeding, sending their diaphragm out of sync with their lungs¹⁵. Just like in adults, hiccups can be silly or annoying, but they’re harmless and will likely go away on their own. They’re certainly not a sign of any infant digestive problems to be worried about or a symptom of overfeeding. As your baby gets older, their hiccupping spells might even become less frequent.

If you notice that your baby seems uncomfortable after feeding, it’s normal to wonder if they’re not getting on with their diet. The likelihood is that their infant digestive system is just developing as normal, but just in case, let’s look at how your baby’s diet can affect their digestion.

Breast milk is a natural laxative, so if you’re breastfeeding your baby, they’ll likely do at least 2 yellowish poos every day for the first 6 weeks¹⁷. After 6 weeks, your breastfed baby might poo less often, and it can be normal for them to go a few days or more without having a poo – it’s very rare for breastfed babies to be constipated¹.

Breastfeeding your baby doesn’t mean they won’t get reflux, colic, or gas. These are all normal as your baby’s digestive system develops and are not necessarily a sign that anything’s wrong.

Digestion can look a little different if your baby is formula-fed, but again, this doesn’t mean that there’s anything to worry about – all first infant formula is created to meet your baby's nutritional needs as they develop over their first year¹⁸.

Early on, your formula-fed baby will poo several times a day¹⁹. As they get older, their poo will start to look firmer, and they might just go once per day¹.

If you bottle-feed your baby, they might swallow some air during feeds, which can cause gas and stomach pain in infants²⁰. Keeping the bottle horizontal but slightly tipped so the teat is always full can help your baby swallow less air.

The vast majority of formula-fed babies are perfectly happy with standard infant-first formula, but if your baby is consistently uncomfortable after feeding, it’s natural to wonder whether their diet might be the cause.

If you have any concerns, it’s always worth speaking to your GP before changing formula. They’ll be able to give you more advice and, if your little one does have a sensitive tummy, perhaps recommend some specific formulas you can use to help.

There isn’t one best baby formula for your baby’s digestive system – they all legally have to include the right nutrition for your baby. A small number of babies might have extra challenges like cow’s milk protein allergy, which can cause stomach discomfort in infants on regular infant formula. In that case, it might be worth exploring some different formula options.

Let’s take a look at some of the different types of formula and when your GP might recommend them.

First infant formula is the ‘standard’ baby formula and should be the first milk you give your baby unless you’re breastfeeding⁵. It’s made with cow’s milk and contains all the nutrients your baby needs. You can give your baby first infant formula until they’re a year old unless a healthcare professional recommends switching. 

Comfort formulas are designed for sensitive tummies to help manage infant digestive symptoms like constipation or colic⁵. The cow’s milk proteins in comfort formula are already partly broken down to help with digestion, but they’re still present, so it’s not suitable if your little one has a cow’s milk protein allergy. Always ask a healthcare professional before switching to a comfort formula.

Anti-reflux formulas are designed for babies who have persistent reflux and struggle to keep any milk down (not for babies who have the odd spit-up). They’re thicker than normal formula, making it easier for babies to keep down⁵. Anti-reflux formulas are suitable from birth, but it’s always best to get advice from a medical professional before switching to one.

Hypoallergenic formula milk is for babies who have a cow’s milk protein allergy. In these formulas, the cow’s milk proteins are fully broken down so that babies with an allergy can digest them⁵. If you think your baby might have a cow’s milk protein allergy, it’s important to get a diagnosis from your healthcare professional so they can prescribe the right formula for you and your little one. 

If you’re thinking of switching infant formula, it’s a good idea to speak to a healthcare professional first. There’s no evidence that changing formulas does your baby any harm, but switching to a formula that your baby doesn’t actually need isn’t recommended.

Speak to your midwife, GP, or health visitor if you have any worries about infant digestive problems. They can give you reassurance and advice, and recommend a different infant formula if needed.

While most infant digestive problems work themselves out as your little one develops, there are some things you can do to gently ease any discomfort.

Whether you breastfeed or bottlefeed your little one, they may swallow little air bubbles during feeds, which can get trapped and make your baby uncomfortable after feeding²¹. If you’re breastfeeding, you can try different positions until you find one that helps your little one latch comfortably and get more milk and less air.

If you feed your baby with a bottle, you can hold it horizontally and slightly tilted to keep the teat full of milk. You can also try paced bottle feeding or using a slow-flow teat to stop the milk from flowing too fast²².

To help your baby’s digestive system get to work, try to keep them upright for around 30 minutes after a feed¹⁵. It’s also a good idea to burp your baby after feeds, or even during if they seem a little fussy²¹. Try different burping positions, including over your shoulder, or sitting on or lying across your lap, to find what works best for your little one.

If your baby still seems unsettled, you can try these gentle techniques to support their digestion:

• Gently massage their tummy in clockwise circles²³.
• Move their legs in a bicycle motion while they’re lying on their back²¹.

The vast majority of newborn stomach discomfort we’ve described here is all part and parcel of your baby’s developing digestive system – they’ll more than likely improve on their own as your baby grows. On very rare occasions, though, stomach discomfort in infants could be a sign of something more serious. 

If you notice any of the following symptoms, you should get in touch with a healthcare professional^24,25,26:

• Greenish vomit or vomiting that’s not normal for your baby.
• A temperature of 38°C or above.
• Signs of dehydration, like fewer wet nappies than usual for your baby or a dry mouth.
• Your baby is lethargic or difficult to wake up.
• Your baby isn’t gaining weight.
• Arching their back when feeding (this can be a sign of gastro-oesophageal reflux disease (GORD)).
• Refusing to feed or extreme distress when feeding.
• Blood in your baby’s poo or wee.
• Persistent diarrhoea. 

Again, these symptoms are very rare, but it’s important to trust your instincts if you have any worries or concerns.

A newborn’s digestive system develops quickly over the first 6 months until it’s mature enough to handle solid foods¹³. After this, their gut continues to develop until late toddlerhood.

There’s no evidence that changing formula will do your baby any harm, but it’s always a good idea to speak to your midwife, GP, or health visitor before making the switch so they can give you some extra advice⁵. 

Yes, it’s normal for your baby to seem a little uncomfortable after a feed as their digestive system develops. They might spit up a little bit of milk, grunt and strain, or hiccup while their body gets used to feeding and digesting^{3, 15, 7}.

Stomach discomfort in infants is often caused by trapped wind. It’s normal for babies to swallow a little air when they’re feeding, which can become trapped as your baby’s little body works out how to relax the right muscles to release it7. You should notice that this gets better as your baby grows.⁷.

If your baby has gas, it should naturally pass on its own or after you’ve burped them. Colic usually lasts longer – when your baby cries for more than 3 hours a day, 3 days in a row for at least a week¹⁰.

Every baby is different, and there’s no one formula that’s easiest to digest. If you decide to formula-feed your baby, you should start with a standard first infant formula. In cases where your GP diagnoses a dietary issue, they may prescribe or refer you to a specialist formula, such as a hypoallergenic formula for cow's milk protein allergy⁵.

Most stomach discomfort in newborns doesn’t last for long. As your baby gets older and their digestive system develops, you’ll likely notice that they grow out of symptoms like reflux and colic naturally^{10, 3}.

When your baby is very little, they have to learn how to relax the muscles in their pelvis to pass wind or have a poo⁷. While they’re figuring this out, it can take a little effort, which is why they might grunt or strain.

Get in touch with a healthcare professional if you notice symptoms like greenish vomit, blood in your baby’s wee or poo or prolonged bouts of diarrhoea, or if your baby doesn’t seem interested in feeding^{24, 25, 26}.

The gut microbiome is made up of all the microbes in your baby’s gut. When they’re born, your baby’s gut microbiome has a lot of developing to do before it reaches maturity. As they start to eat solid foods, it develops and can support digestion and the immune system as well as help balance their mood^{8, 9}.