Hypoplastic Left Heart Syndrome (HLHS)
Medically reviewed by Dr. Michael Okonkwo, MD, FAAP · Board-Certified Neonatologist
Content reviewed against published CDC, AAP, AHA guidelines
Last reviewed:
If your baby has been diagnosed with or you suspect hypoplastic left heart syndrome (hlhs), here is what the evidence says.
The short answer
Hypoplastic left heart syndrome (HLHS) is a critical congenital heart defect where the left side of the heart is severely underdeveloped and cannot effectively pump blood to the body. It is one of the most serious heart defects but can be managed with a series of three surgeries (Norwood, Glenn, and Fontan) or, in some cases, heart transplantation. HLHS is often detected prenatally on fetal echocardiogram or shortly after birth when the baby develops cyanosis, rapid breathing, and poor feeding. Babies with HLHS require immediate specialized care at a pediatric cardiac center.
Key takeaways
- Hypoplastic left heart syndrome (HLHS) is a critical congenital heart defect where the left side of the heart is severely underdeveloped and cannot effectively pump blood to the body. It is one of the most serious heart defects but can be managed with a series of three surgeries (Norwood, Glenn, and Fontan) or, in some cases, heart transplantation. HLHS is often detected prenatally on fetal echocardiogram or shortly after birth when the baby develops cyanosis, rapid breathing, and poor feeding. Babies with HLHS require immediate specialized care at a pediatric cardiac center.
- Usually normal when: Your baby has been evaluated by a pediatric cardiologist and the heart structure is normal
- Call your doctor if: Your newborn has blue or gray skin, rapid breathing, poor feeding, and is lethargic -- these are signs of a critical heart defect and require immediate emergency care (call 911)
- Varies by age — see the age-by-age breakdown below
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What Parents Should Know
According to CDC, AAP, AHA guidelines, hypoplastic left heart syndrome (HLHS) is a critical congenital heart defect where the left side of the heart is severely underdeveloped and cannot effectively pump blood to the body. It is one of the most serious heart defects but can be managed with a series of three surgeries (Norwood, Glenn, and Fontan) or, in some cases, heart transplantation. HLHS is often detected prenatally on fetal echocardiogram or shortly after birth when the baby develops cyanosis, rapid breathing, and poor feeding. Babies with HLHS require immediate specialized care at a pediatric cardiac center. At Prenatal, hLHS is often detected during a routine prenatal ultrasound, typically at the 18-20 week anatomy scan, or on a fetal echocardiogram. Prenatal diagnosis allows parents to plan for delivery at a hospital with a pediatric cardiac surgery program. When diagnosed prenatally, the delivery can be coordinated with a neonatal and cardiac surgical team, which improves outcomes. Genetic counseling may be offered, and parents will meet with the pediatric cardiology team before birth. It is generally considered normal when your baby has been evaluated by a pediatric cardiologist and the heart structure is normal. However, you should contact your pediatrician promptly if your newborn has blue or gray skin, rapid breathing, poor feeding, and is lethargic -- these are signs of a critical heart defect and require immediate emergency care (call 911).
Normal vs. Concerning
When to Seek Immediate Care
- Your newborn has blue or gray skin, rapid breathing, poor feeding, and is lethargic -- these are signs of a critical heart defect and require immediate emergency care (call 911)
- Your baby with HLHS who has had surgery develops new breathing difficulty, is unable to feed, turns blue, or seems very unwell -- go to the emergency room immediately
- Your child with HLHS has a sudden change in their oxygen saturation readings below the range specified by their cardiologist
By Age
What to expect by age
Prenatal
HLHS is often detected during a routine prenatal ultrasound, typically at the 18-20 week anatomy scan, or on a fetal echocardiogram. Prenatal diagnosis allows parents to plan for delivery at a hospital with a pediatric cardiac surgery program. When diagnosed prenatally, the delivery can be coordinated with a neonatal and cardiac surgical team, which improves outcomes. Genetic counseling may be offered, and parents will meet with the pediatric cardiology team before birth.
0-2 weeks
If not detected prenatally, HLHS typically becomes apparent within the first few hours to days of life as the ductus arteriosus (a fetal blood vessel) begins to close. Symptoms include cyanosis (blue or gray skin), rapid or difficult breathing, poor feeding, weak pulse, and lethargy. The baby will be started on prostaglandin medication to keep the ductus arteriosus open and will need the first surgery (Norwood procedure) within the first 1-2 weeks of life.
2 weeks - 6 months
After the Norwood surgery, babies are closely monitored. Parents learn to monitor oxygen saturations, feeding, weight gain, and signs of distress. Between feeds, the baby may need supplemental oxygen or tube feeding. The second surgery (Glenn or hemi-Fontan procedure) is typically performed between 4-6 months of age. This period requires frequent cardiology visits and careful monitoring of growth and heart function.
6 months - 4 years
After the Glenn procedure, babies often show improvement in oxygen levels and feeding. The third and final surgery (Fontan procedure) is typically performed between ages 2-4 years. Between surgeries, children need regular cardiology follow-up, may be on multiple medications, and require monitoring for complications. Despite their heart condition, many children achieve developmental milestones, though some may experience developmental delays.
Long-term
Children who have completed the three-stage surgical palliation for HLHS require lifelong cardiology care. They live with a single-functioning ventricle and may face long-term complications including arrhythmias, protein-losing enteropathy, liver problems, and decreased exercise tolerance. Despite these challenges, many children with HLHS attend school, participate in modified activities, and live meaningful lives. Heart transplant may be needed if the single ventricle begins to fail.
What to Tell Your Pediatrician
- Describe when you first noticed hypoplastic left heart syndrome (hlhs) and how it has changed over time.
- Note your baby's current age and which age-specific patterns you are seeing.
- Mention if a prenatal ultrasound raised any question about the size or structure of the left side of your baby's heart.
- Mention if your baby with known HLHS seems to be feeding less well than usual or gaining weight more slowly.
- 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
- Your baby has been evaluated by a pediatric cardiologist and the heart structure is normal
- Mild variations in heart size seen on prenatal ultrasound that the obstetrician or maternal-fetal medicine specialist determines are within normal limits
- Your child with HLHS is meeting expected post-surgical milestones and their cardiologist is satisfied with their progress
- A prenatal ultrasound raised any question about the size or structure of the left side of your baby's heart
- Your baby with known HLHS seems to be feeding less well than usual or gaining weight more slowly
- Your child with HLHS is more tired than usual or has changes in their typical behavior or activity level
- You have a family history of congenital heart defects and want to discuss prenatal screening
- Your newborn has blue or gray skin, rapid breathing, poor feeding, and is lethargic -- these are signs of a critical heart defect and require immediate emergency care (call 911)
- Your baby with HLHS who has had surgery develops new breathing difficulty, is unable to feed, turns blue, or seems very unwell -- go to the emergency room immediately
- Your child with HLHS has a sudden change in their oxygen saturation readings below the range specified by their cardiologist
What You Can Do at Home
- Keep track of when you notice hypoplastic left heart syndrome (hlhs) — noting the time of day, duration, and any triggers can help your pediatrician.
- Remember that your baby has been evaluated by a pediatric cardiologist and the heart structure is normal — this is generally within the range of normal.
- At Prenatal, 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 blue or gray skin, rapid breathing, poor feeding, and is lethargic -- these are signs of a critical heart defect and require immediate emergency care (call 911).
Related Conditions
Congenital Heart Defect Signs in Babies
Congenital heart defects (CHDs) are the most common type of birth defect, affecting about 1 in 100 babies. They range from small holes in the heart that may close on their own to complex defects requiring multiple surgeries. Early detection through pulse oximetry screening and recognizing key signs — such as blue skin, rapid breathing, poor feeding, and failure to gain weight — is critical. Advances in surgery have dramatically improved survival and quality of life.
My Baby Turns Blue (Cyanosis)
Blue or purple discoloration limited to a baby's hands and feet (acrocyanosis) is very common in newborns and usually harmless, caused by immature circulation. However, blue coloring of the lips, tongue, face, or trunk (central cyanosis) is always a medical emergency that requires immediate evaluation, as it may indicate a heart or lung problem.
Pulse Oximetry Screening in Newborns
Pulse oximetry screening (measuring blood oxygen levels) is done on all newborns before hospital discharge to screen for critical congenital heart disease (CCHD). A normal result is reassuring but does not rule out all heart defects. An abnormal result requires further evaluation but does not necessarily mean there is a problem. Most babies with abnormal results have normal hearts.
Baby Heart Murmur - Innocent vs Concerning
Heart murmurs are very common in children - up to 75% of children will have an audible murmur at some point. The vast majority are "innocent" or "functional" murmurs, meaning there is no structural heart problem. These murmurs are simply the sound of blood flowing normally through the heart and are more audible during fever, illness, or excitement. A smaller number of murmurs indicate a structural heart difference that may need monitoring or treatment. Your pediatrician can usually distinguish between the two and will refer for an echocardiogram if there is any concern.
Related Resources
Frequently asked questions
Is hypoplastic left heart syndrome (hlhs) normal?
When should I call the doctor about hypoplastic left heart syndrome (hlhs)?
When is hypoplastic left heart syndrome (hlhs) normal?
What causes hypoplastic left heart syndrome (hlhs)?
What should I mention to my pediatrician about hypoplastic left heart syndrome (hlhs)?
Is hypoplastic left heart syndrome (hlhs) normal at Prenatal?
Is hypoplastic left heart syndrome (hlhs) normal at 0-2 weeks?
Should I go to the ER for hypoplastic left heart syndrome (hlhs)?
Does hypoplastic left heart syndrome (hlhs) go away on its own?
References
Doctor Visit Checklist
Bring this checklist to your next pediatrician visit to discuss Hypoplastic Left Heart Syndrome (HLHS).
Things to mention
- Describe when you first noticed hypoplastic left heart syndrome (hlhs) and how it has changed over time.
- Note your baby's current age and which age-specific patterns you are seeing.
- Mention if a prenatal ultrasound raised any question about the size or structure of the left side of your baby's heart.
- Mention if your baby with known HLHS seems to be feeding less well than usual or gaining weight more slowly.
- 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
- A prenatal ultrasound raised any question about the size or structure of the left side of your baby's heart
- Your baby with known HLHS seems to be feeding less well than usual or gaining weight more slowly
- Your child with HLHS is more tired than usual or has changes in their typical behavior or activity level
Urgent signs to report immediately
- Your newborn has blue or gray skin, rapid breathing, poor feeding, and is lethargic -- these are signs of a critical heart defect and require immediate emergency care (call 911)
- Your baby with HLHS who has had surgery develops new breathing difficulty, is unable to feed, turns blue, or seems very unwell -- go to the emergency room immediately
- Your child with HLHS has a sudden change in their oxygen saturation readings below the range specified by their cardiologist
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
Bottom line
Most cases of hypoplastic left heart syndrome (hlhs) are normal. Talk to your pediatrician if your newborn has blue or gray skin, rapid breathing, poor feeding, and is lethargic -- these are signs of a critical heart defect and require immediate emergency care (call 911).
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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Related Medical Concerns
Congenital Heart Defect Signs in Babies
Congenital heart defects (CHDs) are the most common type of birth defect, affecting about 1 in 100 babies. They range from small holes in the heart that may close on their own to complex defects requiring multiple surgeries. Early detection through pulse oximetry screening and recognizing key signs — such as blue skin, rapid breathing, poor feeding, and failure to gain weight — is critical. Advances in surgery have dramatically improved survival and quality of life.
My Baby Turns Blue (Cyanosis)
Blue or purple discoloration limited to a baby's hands and feet (acrocyanosis) is very common in newborns and usually harmless, caused by immature circulation. However, blue coloring of the lips, tongue, face, or trunk (central cyanosis) is always a medical emergency that requires immediate evaluation, as it may indicate a heart or lung problem.
Pulse Oximetry Screening in Newborns
Pulse oximetry screening (measuring blood oxygen levels) is done on all newborns before hospital discharge to screen for critical congenital heart disease (CCHD). A normal result is reassuring but does not rule out all heart defects. An abnormal result requires further evaluation but does not necessarily mean there is a problem. Most babies with abnormal results have normal hearts.
Baby Heart Murmur - Innocent vs Concerning
Heart murmurs are very common in children - up to 75% of children will have an audible murmur at some point. The vast majority are "innocent" or "functional" murmurs, meaning there is no structural heart problem. These murmurs are simply the sound of blood flowing normally through the heart and are more audible during fever, illness, or excitement. A smaller number of murmurs indicate a structural heart difference that may need monitoring or treatment. Your pediatrician can usually distinguish between the two and will refer for an echocardiogram if there is any concern.
My Baby's Head Shape Looks Abnormal
Many babies develop temporary head shape irregularities that are completely normal. A cone-shaped head from vaginal delivery reshapes within days. Mild positional flattening (plagiocephaly) from sleeping on the back is very common and usually improves with repositioning and tummy time. However, head shape changes involving ridges, a persistently bulging fontanelle, or rapid head growth changes should be evaluated to rule out craniosynostosis.
Achondroplasia (Dwarfism) in Babies
Achondroplasia is the most common form of short-limbed dwarfism, affecting about 1 in 15,000 to 40,000 births. It is caused by a mutation in the FGFR3 gene and is usually apparent at birth with characteristic features including short limbs, a larger head, and a prominent forehead. Intelligence is normal. With monitoring for specific complications and supportive care, children with achondroplasia lead full, active, and independent lives.