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Green or yellow-green vomit in a baby is an emergency. NICE's postnatal care guideline NG194 names "bilious vomiting (green or yellow-green vomit)" as a red flag for serious illness in young babies, and separately names "frequent forceful (projectile) vomiting". NICE's instruction when any red flag is present is not to observe: arrange an immediate assessment with an appropriate emergency service, and dial 999 if the baby's condition is immediately life-threatening.

Bile in the vomit suggests an obstruction below the stomach. In a newborn that can be a surgical emergency, and delay matters. Call your local emergency number — 999 in the UK and Ireland, 911 in the US and Canada, 000 in Australia, 111 in New Zealand — or go straight to an emergency department.

The AAP's list of vomiting warning signs at any age starts in the same place: "Blood or bile (a green-colored material) in the vomit, or vomit that appears to consist of coffee-ground-like material". The HSE tells parents to call their GP immediately for "bloody, brown, or green vomit".

Vomiting is not posseting

The AAP draws the line clearly: "Vomiting is the forceful throwing up of stomach contents through the mouth. Spitting up (also known as gastroesophageal reflux, or GER) is the easy flow of stomach contents out of the mouth, frequently with a burp."

Posseting is effortless. It runs out, often with a burp, usually within an hour of a feed, and the baby is unbothered. The AAP notes that reflux tends to decrease with age and may persist in a mild form until ten to twelve months, and that it does not interfere with feeding or normal weight gain. The HSE's opening line on the topic is that it is normal for babies to vomit a small amount after feeding.

Vomiting is the opposite: muscular, propulsive, and it looks like the baby is working. Volume is less useful than force as a guide, because a small amount of milk spreads alarmingly.

Pyloric stenosis: the pattern to recognise

The AAP describes this one precisely enough to be useful at home. "Occasional vomiting may occur during a baby's first month. If it appears repeatedly or is unusually forceful, call your pediatrician."

The condition itself: "Hypertrophic pyloric stenosis usually starts at around 2 weeks to 4 months of age. It is caused by a thickening of the muscle at the stomach exit that prevents food from passing into the intestines. This condition requires immediate medical attention. Surgery usually is needed to open the narrowed area."

And the recognisable signature: "The important sign of hypertrophic pyloric stenosis is forceful vomiting occurring approximately 15 to 30 minutes or less after every feeding. Anytime you notice this, call your pediatrician as soon as possible."

The picture parents describe is a baby who vomits hard, very soon after feeding, then is immediately hungry again — and who over days stops gaining weight, has fewer wet nappies and becomes constipated, because very little is getting through. The vomit is milk, not bile, because the blockage is above where bile enters. That is why "not green" is not reassuring here.

The other causes worth knowing

Infection. The AAP notes that once a baby is a few months old, the commonest cause of vomiting is a stomach or intestinal infection, and that infections outside the gut — urinary tract, chest, ears, meningitis — also cause vomiting. In a newborn, vomiting can be one of the few outward signs of infection, which is why fever thresholds matter so much at this age.
Cow's milk protein allergy. The AAP notes symptoms overlapping with reflux, managed by removing cow's milk and dairy from a breastfeeding parent's diet or changing to a formula with broken-down protein, on medical advice.
Reflux disease. The AAP describes GERD as spitting up that gets worse rather than better and happens all the time, associated with pain on feeding, gagging or coughing, and poor weight gain.
Obstruction. The HSE lists pyloric stenosis, intussusception and strangulated hernia among the more serious causes of vomiting in babies.

Dehydration is the thing that gets you

Newborns dehydrate quickly. The HSE lists the signs of severe dehydration in babies as fewer wet nappies, drowsiness or being hard to wake, pale or blotchy skin, and cold hands and feet, and says to go to the nearest emergency department immediately if any are present. The AAP's dehydration list adds a dry mouth, absent tears and depression of the soft spot.

Two more thresholds from the HSE, which are firm numbers rather than impressions. Call your GP immediately if your child has vomited more than three times in the past 24 hours, has been vomiting for more than three days, shows signs of dehydration, has severe tummy pain, or has a temperature of 38C in a baby under three months, or 39C over three months.

One instruction the HSE gives that is easy to get wrong: it is not safe to dilute baby formula. Make bottles up as normal. If you are breastfeeding, keep feeding — a sick baby often wants more frequent, shorter feeds, and the HSE says there is no need to give additional fluids.

When to call

Call your local emergency number (999 in the UK and Ireland, 911 in the US and Canada, 000 in Australia, 111 in New Zealand) if your baby:

Vomits green or yellow-green, blood, or anything like coffee grounds.
Vomits forcefully after most or every feed.
Has a headache-like arching with a stiff neck and a rash alongside vomiting — the HSE names this combination specifically as a 999 call.
Has a temperature of 38C (100.4F) or above, or below 36C (96.8F).
Looks pale, ashen, mottled or blue, is unresponsive or unrousable, or has a weak, abnormally high-pitched or continuous cry.
Is grunting, breathing over 60 breaths a minute, or drawing their chest in.
Has a bulging fontanelle, a seizure, or a rash that does not fade under a pressed glass.
Shows signs of severe dehydration: far fewer wet nappies, drowsiness, blotchy skin, cold hands and feet, a sunken soft spot.

Contact your GP, midwife, health visitor or public health nurse the same day if: the vomiting is repeated but not forceful; your baby is refusing feeds; weight gain has stalled; there is a swollen or hard tummy; your baby is unusually irritable or unusually sleepy; or the vomiting has gone on beyond 24 hours. The AAP names a swollen abdomen, lethargy or severe irritability, jaundice and inability to drink enough as reasons to call whatever the age.

What to let go of

Do not spend the evening deciding whether what you saw counts as projectile. Force, colour and your baby's condition are the three things worth reporting, and any one of the red flags above ends the decision for you. NICE's own warning about young babies is the reason to lean early: the presence or absence of individual symptoms may be of limited value in ruling out serious illness at this age.

Newborn Projectile and Green Vomiting