Medical Conditions

Maple Syrup Urine Disease (MSUD) in Babies

Medically reviewed by Dr. Michael Okonkwo, MD, FAAP · Board-Certified Neonatologist

Content reviewed against published NIH, NORD, March of Dimes guidelines

Editorial policy

Last reviewed:

If your baby has been diagnosed with or you suspect maple syrup urine disease (msud) in babies, here is what the evidence says.

The short answer

Maple syrup urine disease (MSUD) is a rare inherited metabolic disorder in which the body cannot properly break down three amino acids: leucine, isoleucine, and valine (branched-chain amino acids). It is named for the characteristic sweet, maple syrup-like odor of the urine. MSUD is detected through newborn screening, and with prompt dietary treatment started within the first days of life, babies with MSUD can develop normally. False positives on newborn screening are possible, so a positive screen requires confirmatory testing.

Key takeaways

  • Maple syrup urine disease (MSUD) is a rare inherited metabolic disorder in which the body cannot properly break down three amino acids: leucine, isoleucine, and valine (branched-chain amino acids). It is named for the characteristic sweet, maple syrup-like odor of the urine. MSUD is detected through newborn screening, and with prompt dietary treatment started within the first days of life, babies with MSUD can develop normally. False positives on newborn screening are possible, so a positive screen requires confirmatory testing.
  • Usually normal when: Your baby's newborn screening came back normal for MSUD
  • Call your doctor if: Your newborn has a positive MSUD screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as MSUD can cause a life-threatening metabolic crisis within days
  • Varies by age — see the age-by-age breakdown below
“Fever itself is not an illness — rather, it is a sign or symptom that the body is fighting an infection. Fever stimulates certain defenses, such as the white blood cells, which attack and destroy invading bacteria.”
— Fever and Your Child, American Academy of Pediatrics (AAP)

Parents everywhere have the same worry. You are doing the right thing by looking into it.

What Parents Should Know

According to NIH, NORD, March of Dimes guidelines, maple syrup urine disease (MSUD) is a rare inherited metabolic disorder in which the body cannot properly break down three amino acids: leucine, isoleucine, and valine (branched-chain amino acids). It is named for the characteristic sweet, maple syrup-like odor of the urine. MSUD is detected through newborn screening, and with prompt dietary treatment started within the first days of life, babies with MSUD can develop normally. False positives on newborn screening are possible, so a positive screen requires confirmatory testing. At 0-1 week, babies with MSUD appear healthy at birth. Symptoms typically begin within 2-3 days as branched-chain amino acids accumulate. Without treatment, newborns may develop poor feeding, vomiting, lethargy, and a distinctive sweet smell to their urine, earwax, and sweat. Newborn screening usually detects MSUD before symptoms become severe. If your baby's screening is positive, confirmatory blood tests (plasma amino acids) will be done urgently, and a special formula free of leucine, isoleucine, and valine is started immediately. It is generally considered normal when your baby's newborn screening came back normal for MSUD. However, you should contact your pediatrician promptly if your newborn has a positive MSUD screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as MSUD can cause a life-threatening metabolic crisis within days.

Sources: [1], [2], [3]

Normal vs. Concerning

Usually Normal
Worth Discussing
Your baby's newborn screening came back normal for MSUD
Your newborn has a positive MSUD screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as MSUD can cause a life-threatening metabolic crisis within days
Your baby is on MSUD treatment and leucine levels are consistently in the target range
Your baby with MSUD is vomiting, refusing to eat, unusually sleepy, or has a sweet maple syrup smell — these may indicate a metabolic crisis requiring emergency treatment
Your treated baby is meeting developmental milestones on schedule
Your baby with MSUD has any change in alertness, abnormal movements, or difficulty breathing — go to the emergency room immediately
A positive newborn screen was followed by normal confirmatory testing — false positives do occur
You received an abnormal or borderline MSUD result on your baby's newborn screen and are awaiting confirmatory testing

When to Seek Immediate Care

  • Your newborn has a positive MSUD screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as MSUD can cause a life-threatening metabolic crisis within days
  • Your baby with MSUD is vomiting, refusing to eat, unusually sleepy, or has a sweet maple syrup smell — these may indicate a metabolic crisis requiring emergency treatment
  • Your baby with MSUD has any change in alertness, abnormal movements, or difficulty breathing — go to the emergency room immediately

By Age

What to expect by age

0-1 week

Babies with MSUD appear healthy at birth. Symptoms typically begin within 2-3 days as branched-chain amino acids accumulate. Without treatment, newborns may develop poor feeding, vomiting, lethargy, and a distinctive sweet smell to their urine, earwax, and sweat. Newborn screening usually detects MSUD before symptoms become severe. If your baby's screening is positive, confirmatory blood tests (plasma amino acids) will be done urgently, and a special formula free of leucine, isoleucine, and valine is started immediately.

1-6 months

Babies on a properly managed MSUD diet grow and develop normally. Blood levels of branched-chain amino acids are monitored very frequently — often twice weekly in the early months — to keep leucine levels in the safe range. Your metabolic team will carefully titrate the amount of regular formula or breast milk mixed with the special MSUD formula. During illnesses, even minor ones like a cold, leucine levels can spike dangerously, so your team will provide a sick-day protocol.

6-12 months

Introducing solid foods requires careful planning with your metabolic dietitian. Most fruits and some vegetables are naturally low in branched-chain amino acids and can be introduced. High-protein foods like meat, dairy, and legumes must be carefully measured or avoided. Your baby should continue to meet developmental milestones on schedule with well-managed treatment. Leucine levels are still monitored regularly.

1-3 years

MSUD is a lifelong condition requiring strict dietary management. Toddlers can be challenging because of food preferences and refusal, but maintaining the diet is critical to prevent brain damage. Metabolic crises can occur during illness, fasting, or stress. Some families keep emergency supplies of a leucine-free formula for sick days. Liver transplantation is a potential option that can allow an unrestricted diet, though it carries its own risks.

What to Tell Your Pediatrician

  • Describe when you first noticed maple syrup urine disease (msud) in babies and how it has changed over time.
  • Note your baby's current age and which age-specific patterns you are seeing.
  • Mention if you received an abnormal or borderline MSUD result on your baby's newborn screen and are awaiting confirmatory testing.
  • Mention if your baby on MSUD treatment has leucine levels that are running above target.
  • Let your doctor know if you have noticed any related concerns, such as changes in feeding, sleep, or movement patterns.
  • Bring a list of any questions or observations you want to discuss at the appointment.

What Should You Do?

When to take action

Probably normal when...
  • Your baby's newborn screening came back normal for MSUD
  • Your baby is on MSUD treatment and leucine levels are consistently in the target range
  • Your treated baby is meeting developmental milestones on schedule
  • A positive newborn screen was followed by normal confirmatory testing — false positives do occur
Mention at your next visit when...
  • You received an abnormal or borderline MSUD result on your baby's newborn screen and are awaiting confirmatory testing
  • Your baby on MSUD treatment has leucine levels that are running above target
  • You are struggling to maintain the MSUD diet and need additional support from your metabolic team
  • Your baby with MSUD is developing a minor illness and you need guidance on the sick-day protocol
Act now when...
  • Your newborn has a positive MSUD screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as MSUD can cause a life-threatening metabolic crisis within days
  • Your baby with MSUD is vomiting, refusing to eat, unusually sleepy, or has a sweet maple syrup smell — these may indicate a metabolic crisis requiring emergency treatment
  • Your baby with MSUD has any change in alertness, abnormal movements, or difficulty breathing — go to the emergency room immediately

What You Can Do at Home

  • Keep track of when you notice maple syrup urine disease (msud) in babies — noting the time of day, duration, and any triggers can help your pediatrician.
  • Remember that your baby's newborn screening came back normal for MSUD — this is generally within the range of normal.
  • At 0-1 week, focus on observation rather than intervention unless your pediatrician advises otherwise.
  • Follow any care instructions from your pediatrician. Keep a written log of symptoms to bring to appointments.
  • While monitoring at home, seek immediate care if your newborn has a positive MSUD screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as MSUD can cause a life-threatening metabolic crisis within days.

Abnormal Newborn Screening Results

An abnormal or positive newborn screening result means that further testing is needed, not that your baby definitely has a condition. Most positive screens turn out to be false positives after confirmatory testing. Newborn screening tests are intentionally sensitive to avoid missing any affected babies, so follow-up testing is essential to determine whether the result is a true positive.

Newborn Metabolic Screening Results

The newborn metabolic screen (heel prick blood test) checks for dozens of rare but treatable conditions including metabolic disorders, endocrine disorders, and hemoglobinopathies. Most babies have normal results. An abnormal result requires follow-up testing but does not mean your baby definitely has a condition. Many initial abnormal results turn out to be false positives.

Signs of a Metabolic Disorder in Babies

Metabolic disorders (inborn errors of metabolism) occur when the body cannot properly process certain nutrients or chemicals. Many are detected through newborn screening. Signs can include poor feeding, vomiting, lethargy, seizures, unusual body or urine odor, failure to thrive, developmental regression, and metabolic crises triggered by illness or fasting. Early detection and treatment are crucial, as many metabolic disorders are manageable with dietary modifications and medical treatment.

Milestone

Month-by-Month Development

Detailed monthly development guides from birth through 24 months.

Guide

When to Call the Doctor

General guide on when to call the pediatrician, visit urgent care, or go to the ER.

Frequently asked questions

Is maple syrup urine disease (msud) in babies normal?
Maple syrup urine disease (MSUD) is a rare inherited metabolic disorder in which the body cannot properly break down three amino acids: leucine, isoleucine, and valine (branched-chain amino acids). It is named for the characteristic sweet, maple syrup-like odor of the urine. MSUD is detected through newborn screening, and with prompt dietary treatment started within the first days of life, babies with MSUD can develop normally. False positives on newborn screening are possible, so a positive screen requires confirmatory testing.
When should I call the doctor about maple syrup urine disease (msud) in babies?
Your newborn has a positive MSUD screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as MSUD can cause a life-threatening metabolic crisis within days Your baby with MSUD is vomiting, refusing to eat, unusually sleepy, or has a sweet maple syrup smell — these may indicate a metabolic crisis requiring emergency treatment Your baby with MSUD has any change in alertness, abnormal movements, or difficulty breathing — go to the emergency room immediately
When is maple syrup urine disease (msud) in babies normal?
Your baby's newborn screening came back normal for MSUD Your baby is on MSUD treatment and leucine levels are consistently in the target range Your treated baby is meeting developmental milestones on schedule
What causes maple syrup urine disease (msud) in babies?
Maple syrup urine disease (MSUD) is a rare inherited metabolic disorder in which the body cannot properly break down three amino acids: leucine, isoleucine, and valine (branched-chain amino acids). It is named for the characteristic sweet, maple syrup-like odor of the urine. MSUD is detected through newborn screening, and with prompt dietary treatment started within the first days of life, babies with MSUD can develop normally. False positives on newborn screening are possible, so a positive screen requires confirmatory testing. Common explanations include: Your baby's newborn screening came back normal for MSUD. Your baby is on MSUD treatment and leucine levels are consistently in the target range.
What should I mention to my pediatrician about maple syrup urine disease (msud) in babies?
You should mention maple syrup urine disease (msud) in babies at your next visit if: You received an abnormal or borderline MSUD result on your baby's newborn screen and are awaiting confirmatory testing. Your baby on MSUD treatment has leucine levels that are running above target. You are struggling to maintain the MSUD diet and need additional support from your metabolic team.
Is maple syrup urine disease (msud) in babies normal at 0-1 week?
Babies with MSUD appear healthy at birth. Symptoms typically begin within 2-3 days as branched-chain amino acids accumulate. Without treatment, newborns may develop poor feeding, vomiting, lethargy, and a distinctive sweet smell to their urine, earwax, and sweat. Newborn screening usually detects MSUD before symptoms become severe. If your baby's screening is positive, confirmatory blood tests (plasma amino acids) will be done urgently, and a special formula free of leucine, isoleucine, and valine is started immediately.
Is maple syrup urine disease (msud) in babies normal at 1-6 months?
Babies on a properly managed MSUD diet grow and develop normally. Blood levels of branched-chain amino acids are monitored very frequently — often twice weekly in the early months — to keep leucine levels in the safe range. Your metabolic team will carefully titrate the amount of regular formula or breast milk mixed with the special MSUD formula. During illnesses, even minor ones like a cold, leucine levels can spike dangerously, so your team will provide a sick-day protocol.
Should I go to the ER for maple syrup urine disease (msud) in babies?
Seek emergency care if your newborn has a positive MSUD screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as MSUD can cause a life-threatening metabolic crisis within days, or if your baby with MSUD is vomiting, refusing to eat, unusually sleepy, or has a sweet maple syrup smell — these may indicate a metabolic crisis requiring emergency treatment. When in doubt, call your pediatrician's after-hours line for guidance.
Does maple syrup urine disease (msud) in babies go away on its own?
In many cases, maple syrup urine disease (msud) in babies resolves on its own, especially when your baby's newborn screening came back normal for MSUD. By 1-3 years, mSUD is a lifelong condition requiring strict dietary management. Toddlers can be challenging because of food preferences and refusal, but maintaining the diet is critical to prevent brain damage. Metabolic crises can occur during illness, fasting, or stress. Some families keep emergency supplies of a leucine-free formula for sick days. Liver transplantation is a potential option that can allow an unrestricted diet, though it carries its own risks.
NIHNORDMARCH OF DIMES

References

  1. [1]National Library of Medicine. Maple Syrup Urine Disease. MedlinePlus Genetics, 2023. NIH
  2. [2]National Organization for Rare Disorders. Maple Syrup Urine Disease. NORD, 2023. NORD
  3. [3]March of Dimes. Newborn Screening Tests for Your Baby. March of Dimes, 2023. March of Dimes

Doctor Visit Checklist

Bring this checklist to your next pediatrician visit to discuss Maple Syrup Urine Disease (MSUD) in Babies.

Things to mention

  • Describe when you first noticed maple syrup urine disease (msud) in babies and how it has changed over time.
  • Note your baby's current age and which age-specific patterns you are seeing.
  • Mention if you received an abnormal or borderline MSUD result on your baby's newborn screen and are awaiting confirmatory testing.
  • Mention if your baby on MSUD treatment has leucine levels that are running above target.
  • Let your doctor know if you have noticed any related concerns, such as changes in feeding, sleep, or movement patterns.
  • Bring a list of any questions or observations you want to discuss at the appointment.

Observations to share

  • You received an abnormal or borderline MSUD result on your baby's newborn screen and are awaiting confirmatory testing
  • Your baby on MSUD treatment has leucine levels that are running above target
  • You are struggling to maintain the MSUD diet and need additional support from your metabolic team

Urgent signs to report immediately

  • Your newborn has a positive MSUD screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as MSUD can cause a life-threatening metabolic crisis within days
  • Your baby with MSUD is vomiting, refusing to eat, unusually sleepy, or has a sweet maple syrup smell — these may indicate a metabolic crisis requiring emergency treatment
  • Your baby with MSUD has any change in alertness, abnormal movements, or difficulty breathing — go to the emergency room immediately

My notes

From ismybabyalright.com — free, evidence-based baby health guides

All content follows our editorial policy and is reviewed against published clinical guidelines.

2,705 evidence-based guides6 authoritative medical sources5 medical advisory board members

Related Resources

Well-Baby Visit PrepEmergency ContactsBaby Tracker

Bottom line

Most cases of maple syrup urine disease (msud) in babies are normal. Talk to your pediatrician if your newborn has a positive msud screen and you have not yet been contacted by a metabolic specialist — call your pediatrician immediately, as msud can cause a life-threatening metabolic crisis within days.

Trust your instincts. If something feels wrong, reach out to your pediatrician. Worrying about your baby means you care — that is a good thing.

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An abnormal or positive newborn screening result means that further testing is needed, not that your baby definitely has a condition. Most positive screens turn out to be false positives after confirmatory testing. Newborn screening tests are intentionally sensitive to avoid missing any affected babies, so follow-up testing is essential to determine whether the result is a true positive.

Newborn Metabolic Screening Results

The newborn metabolic screen (heel prick blood test) checks for dozens of rare but treatable conditions including metabolic disorders, endocrine disorders, and hemoglobinopathies. Most babies have normal results. An abnormal result requires follow-up testing but does not mean your baby definitely has a condition. Many initial abnormal results turn out to be false positives.

Signs of a Metabolic Disorder in Babies

Metabolic disorders (inborn errors of metabolism) occur when the body cannot properly process certain nutrients or chemicals. Many are detected through newborn screening. Signs can include poor feeding, vomiting, lethargy, seizures, unusual body or urine odor, failure to thrive, developmental regression, and metabolic crises triggered by illness or fasting. Early detection and treatment are crucial, as many metabolic disorders are manageable with dietary modifications and medical treatment.

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