Blood in a Baby's Diaper: Where It Can Come From and When It's Urgent

If none of those fit, take a breath.

Most red in a diaper is something small, and a good share of it isn't blood at all: Seattle Children's puts it at 90% of red stools in children. In a baby under 12 weeks, though, any blood in the stool means calling your doctor today, even when the baby seems fine.

I'm a physiotherapist, which means I know bodies and movement, not bowels, so everything clinical below comes from the pediatric sources listed at the end. What I bring from five kids is the other part: I've stood over a changing table at 6am trying to decide whether a pink smudge was an emergency, and I know how hard thinking straight gets at that hour.

Blood in the stool

Where the blood is tells you a lot. A thin bright red streak on the outside of a firm poop usually comes from the very end of the bowel, most often a small tear. Blood mixed through with mucus, in loose stools, points somewhere a little higher, and black, tarry stool means blood that's been through the stomach. Those three get different answers, so look closely before you fold the diaper.

Anal fissures

A fissure is a small tear at the opening of the bottom, usually caused by passing a large or hard stool, and it's the cause in about 90% of children with blood in their stools. You'll typically see a streak or a few spots of bright red on the outside of the poop or on the wipe, in a baby who's otherwise their normal self. It stings, too. Some babies cry as the stool passes, and that's often the first clue. Most tears heal quickly. Seattle Children's suggests a warm-water soak for about 20 minutes, and calling the doctor if the bleeding gets worse or happens more than twice. For a baby, I'd ask before putting any cream on the area, and I'd mention any hard stools on the same call, since the constipation is the thing to fix.

Streaks with mucus in a young baby

In the first couple of months, another familiar cause is allergic proctocolitis, an inflammation of the lower bowel set off by a food protein, usually cow's milk and sometimes soy. A 2020 review describes the typical baby as healthy-looking and gaining weight, with red blood and mucus mixed into the stool, sometimes with loose stools. More than half of reported cases are exclusively breastfed babies, reacting to protein that reaches them through breast milk. When the trigger is removed, visible bleeding usually settles within 72 to 96 hours, and nearly all babies grow out of it by one to three years old.

Please don't start cutting foods on your own, though. The same review is clear that infection, fissures, intussusception and, in newborns, a serious bowel condition called necrotizing enterocolitis have to be ruled out first, and only a doctor can do that.

Swallowed blood

Babies can pass red or dark stool after swallowing their mother's blood, either during delivery or from a cracked nipple while breastfeeding. A UK emergency medicine teaching page adds a detail I didn't know: cracked nipples more often show up as blood in a well baby's milky spit-up than in the stool, and blood swallowed around the birth tends to come out dark. Your doctor can check whether the blood is yours or the baby's, so mention any nipple bleeding when you call.

A newborn girl's mini-period

This one frightens a lot of parents and is usually completely normal. Before birth, a baby girl is exposed to her mother's estrogen, and when those levels drop quickly after delivery she can have a small amount of vaginal bleeding during the first one to two weeks. It's often just a pink or red smear mixed with a little white discharge, at the front of the diaper and not in the poop.

Bleeding that lasts more than a week, starts after the first week, or is more than a small amount needs a doctor. After the newborn weeks, any vaginal bleeding in a baby should be seen right away.

Urate crystals, which aren't blood

In the first few days you may find reddish pink crystals in a wet diaper, often described as looking like brick dust. They're urates, salts from concentrated urine, and they should stop appearing as your baby starts taking more milk. They look powdery, they're where the urine soaked in, and they aren't mixed into stool. If they keep turning up after the first few days, or the diapers aren't getting heavy and wet, call your midwife, lactation consultant or doctor, because it can mean the baby needs help feeding more.

Not sure whether the red came from the poop or the urine? The AAP suggests a towel test for an older baby: lie them on a towel without a diaper for a while and see where the color shows up. For a newborn, a photo and a quick call are simpler.

Blood in the urine

Pink, red or tea-colored urine is on Seattle Children's list for calling the doctor or seeking care now. Foods, food dyes and some medicines can tint urine too, and in a newborn the urates above are the usual explanation for pink. Real blood in the urine with a fever, or crying when your baby pees, can point to a urinary infection, and that's something a young baby needs seen promptly.

Bleeding from a raw diaper rash

A badly inflamed rash can crack and leave pinpoint spots of blood on the diaper, and that blood is on the skin, not in the poop or urine. The AAP's symptom checker says to contact your doctor during office hours if a rash is very raw or bleeds, and within 24 hours for blisters, pimples, yellow scabs or open sores. Seek care now if the skin is bright red and peeling off in sheets, if there's a fever with spreading redness, or if a baby under one month has a rash that looks infected or has tiny blisters in a group.

Black or tarry stool

In the first day or two, black and tarry is exactly right: that's meconium, and around days three to five it should change to dark green and then yellow. After that, black stool needs a closer look. Dark green is a normal color that can pass for black in a dim room, so Seattle Children's suggests smearing a little on white paper under a bright light, and iron medicine can darken stool as well. Black, tarry stool that isn't explained by either is on the seek-care-now list, because it can mean bleeding higher in the gut.

One rare cause belongs here because it's so serious. Vitamin K deficiency bleeding happens when a baby doesn't have enough vitamin K to clot, and the CDC lists blood in the stool or black, tarry stool among its warning signs, along with bruising, bleeding from the nose or cord, and unusual sleepiness. Babies who didn't have the vitamin K shot at birth can develop it at any point up to six months, and it often doesn't give any warning before a serious bleed, so if your baby missed the shot, any of those signs means the emergency department.

Red after solids

Once food starts, so do the false alarms. Beets, watermelon, tomato skins, red food coloring and dark grape juice can all turn stool red, and red liquid medicines such as some antibiotics can too. If you can name the food, stop it and watch: the AAP says the color should clear within about 48 hours. If it lasts longer than that, or comes with a fever or a baby who seems sick, that's a reason to call. My fourth once produced a diaper after a beet puree that I photographed before I'd even put her down, and I'm not too proud to say I phoned anyway.

What to bring to the doctor

Keep the diaper. Seattle Children's tells parents to save a sample of the blood for testing, so fold it into a bag instead of the bin. Take a photo in daylight too, as the AAP suggests, since color shifts under a lamp. Note when you last saw a normal stool, how feeds and wet diapers have been, any fever, and anything new: a formula change, a first food, a medicine, or cracked nipples. That list takes two minutes to write and saves a lot of guessing in the room.

Intussusception, the telescoping of one part of the bowel into the next, is most common between three months and two years, and the redcurrant-jelly stool is often a later sign that follows bouts of severe pain and vomiting. If those bouts come back after you've been seen, Cambridge's children's team says to go straight to the nearest hospital.

What the research says

• Seattle Children's: Stools, blood in
• Seattle Children's: Stools, unusual color
• American Academy of Pediatrics: Red stools in children, common causes
• American Academy of Pediatrics symptom checker: Diaper rash
• RCEM Learning: Neonate with blood in the nappy, look beyond the cracked nipples
• La Leche League GB: What's in a nappy?
• MSD Manual: Vaginal bleeding in infants and children
• Mennini et al. (2020), Food protein-induced allergic proctocolitis in infants, World Allergy Organ J
• CDC: About vitamin K deficiency bleeding
• Cambridge University Hospitals NHS: Intussusception in children
• KidsHealth: Blood in the urine (hematuria)
• AboutKidsHealth: Meconium

This guide is general information, written to help you understand a topic and ask better questions. It is not medical advice and does not replace your own doctor, midwife or paediatrician. Anything that worries you about your baby belongs with them.

Nearby

• What Should Newborn Poop Look Like? A Colour and Texture Guide
• Why Is My Baby's Poop Green?
• What Does Diaper Rash Look Like?

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