Blood in Baby Stool: Causes, What Each Color Means, and When to Call the Doctor
Bright red streaks usually indicate an anal fissure — the most common cause. Mucousy bloody stool may signal cow's milk protein allergy. Black tarry stool is urgent. Always call your pediatrician the same day.
🔴 Bright Red Streaks: Anal Fissure (Most Common)
The most common cause of blood in a baby's stool is an anal fissure — a small tear in the delicate skin around the anus, usually caused by passing a firm or large stool. This is especially common in formula-fed babies and babies who have recently started solids.
- Blood appears as bright red streaks on the outside of the stool, or as small spots on the diaper — not mixed into the stool itself
- The baby may cry or strain more than usual during bowel movements because the fissure stings as stool passes over it
- You may be able to see a tiny crack or raw-looking area at the edge of the anus
- Treatment focuses on resolving the underlying constipation — dietary changes, prune juice (for babies 6+ months), or pediatrician-recommended stool softeners
- Applying a thin layer of petroleum jelly (Vaseline) to the area can protect the fissure and help it heal, usually within a few days
- Fissures tend to recur if constipation isn't addressed — work with your pediatrician on a long-term plan for softer stools
🥛 Mucousy + Bloody Stool: Cow's Milk Protein Allergy
Cow's milk protein allergy (CMPA) affects an estimated 2-3% of infants and is the second most common cause of blood in baby stool. It can occur in both formula-fed babies (direct exposure) and breastfed babies (cow's milk proteins pass through breast milk when the mother consumes dairy).
- Stool often appears mucousy — like it has a coating of clear or greenish jelly — with visible streaks or flecks of blood mixed in
- Other signs include increased fussiness (especially during or after feeds), eczema or skin rashes, frequent spitting up, and occasionally poor weight gain
- Breastfed babies: The mother eliminates all dairy from her diet for 2-4 weeks — it takes about 2 weeks for cow's milk protein to fully clear from breast milk. Improvement in stool is typically seen within 72 hours to 2 weeks
- Formula-fed babies: Switch to an extensively hydrolyzed formula (e.g., Nutramigen or Alimentum) where the proteins are broken down, or in severe cases, an amino acid-based formula (e.g., EleCare or Neocate)
- About 50% of babies with CMPA also react to soy protein, so soy formula is not always a reliable alternative
- Most babies outgrow CMPA by 12-18 months — your pediatrician will guide reintroduction
🤱 Swallowed Maternal Blood
If you're breastfeeding and have cracked, bleeding, or damaged nipples, your baby may swallow small amounts of your blood during nursing. This is a benign cause that resolves once your nipples heal, but it needs to be confirmed by your pediatrician.
- Blood may appear as dark flecks or streaks in the stool, or baby may spit up material that looks like coffee grounds
- The Apt test (Apt-Downey test) can be performed in the pediatrician's office to determine if the blood is adult (maternal) or fetal (baby's) hemoglobin
- If swallowed maternal blood is confirmed, the priority is treating the nipple damage — work with a lactation consultant on latch improvement, nipple shields if needed, and healing with expressed milk or medical-grade lanolin
- Swallowed blood during delivery can also cause dark-colored stools in the first few days of life — this resolves as meconium transitions to normal stool
🦠 Infection-Related Blood
Bacterial or viral gastrointestinal infections can cause bloody diarrhea in babies. This is more common in babies who attend daycare, have been exposed to contaminated food or water, or have recently taken antibiotics.
- Bacterial infections (Salmonella, Campylobacter, E. coli, C. difficile) can cause bloody diarrhea, often with fever and vomiting
- Your pediatrician will likely request a stool culture to identify the specific organism
- Dehydration is the primary concern with infectious diarrhea — watch for fewer wet diapers, dry mouth, no tears when crying, and sunken fontanelle
- Most bacterial intestinal infections in babies resolve without antibiotics, but severe cases may require treatment
- C. difficile colitis can occur after antibiotic use — mention any recent antibiotics to your pediatrician
🚨 When to Go to the Emergency Room
While most blood in baby stool has a benign explanation, certain presentations require immediate medical evaluation.
- Red currant jelly stool: Stool that looks like red jelly or jam may indicate intussusception (the intestine telescoping into itself) — this is a medical emergency most common between 6-36 months, often accompanied by sudden severe abdominal pain that comes in waves
- Large volume of blood: More than just streaks — if the diaper looks bloody or there are clots
- Black tarry stool after the meconium period (first 3-4 days of life)
- Baby appears ill: Lethargy, difficulty waking, pale skin, rapid breathing, high fever, or refusal to feed alongside bloody stool
- Distended, rigid abdomen that appears swollen and is hard to the touch
- Bilious (green) vomiting with bloody stool — possible sign of bowel obstruction
📋 What Your Pediatrician Will Do
When you report blood in your baby's stool, your pediatrician will take a thorough history and may perform several tests to determine the cause.
- Feeding history: Breast vs. formula, any recent formula changes, solids introduction, and maternal diet if breastfeeding
- Stool sample (guaiac test): Confirms whether visible discoloration is actually blood — some foods and medications can mimic blood
- Rectal exam: Checks for fissures, which are the most common cause
- Stool culture: If infection is suspected, identifies bacterial pathogens
- Apt test: Differentiates maternal from infant blood if swallowed maternal blood is suspected
- Allergy workup: A 2-4 week dairy elimination trial is the gold standard for diagnosing CMPA — blood tests for food allergies are unreliable in infants
- Take a photo of the diaper before your appointment — stool appearance changes over time and a visual reference is extremely helpful for diagnosis