Hypoplastic left heart syndrome (HLHS) has no single cause. It results from a combination of genetic factors, disrupted blood flow during fetal development, and in some cases, environmental exposures during pregnancy. About 1 in 3,846 babies in the United States are born with HLHS each year, making it rare but one of the most serious congenital heart defects.
Most cases appear spontaneously in families with no prior history of heart defects. But research over the past two decades has identified several genetic, developmental, and environmental factors that raise the risk.
How HLHS Develops Before Birth
HLHS occurs when the left side of the heart, including the left ventricle, the mitral valve, and the aortic valve, fails to grow properly during fetal development. The prevailing theory is sometimes called “no flow, no grow”: if blood doesn’t flow normally through the left side of the heart early in pregnancy, the structures on that side stop developing. A valve that forms too narrowly or doesn’t open at all means less blood passes through, and the left ventricle never receives the mechanical stimulation it needs to grow to full size.
This can start as a relatively minor valve abnormality in the first trimester that progressively worsens. As the fetus grows and the heart remodels, the underdeveloped left side falls further and further behind. By birth, the left ventricle is too small to pump blood to the body, and the baby depends entirely on the right side of the heart and a small temporary blood vessel (the ductus arteriosus) to survive.
Genetic Mutations Linked to HLHS
Several specific genes play a role. Mutations in a gene called NOTCH1 are strongly associated with developmental aortic valve problems, and research has identified novel NOTCH1 mutations in families where multiple members have single-ventricle heart defects like HLHS. This was some of the first direct human genetic evidence that a specific gene mutation could cause congenital heart disease. Another gene, NKX2.5, which helps direct early heart formation, is also associated with HLHS when it carries certain variants.
These aren’t simple inherited mutations in most cases. The majority of HLHS occurs sporadically, meaning the genetic changes happen for the first time in the affected child rather than being passed down from a parent who also had the condition. However, the genetic component is real: parents who have one child with HLHS face roughly a 2% recurrence risk in future pregnancies. Some families appear to carry autosomal recessive patterns, where both parents silently carry a gene variant. A case report of three siblings all born with isolated HLHS suggests the recurrence risk may sometimes be higher than 2%, particularly in families with a clear hereditary pattern.
Chromosomal Abnormalities and Syndromes
Up to 20% of HLHS cases occur alongside a chromosomal abnormality or genetic syndrome. Turner syndrome (where a female is missing one X chromosome), trisomy 13, and trisomy 18 are the most commonly associated conditions. In these cases, HLHS is one of several developmental problems caused by the broader chromosomal issue. When HLHS is detected on a prenatal ultrasound, genetic testing is typically offered to check for these conditions, since they significantly affect the overall prognosis and treatment decisions.
Maternal Diabetes and Pregnancy Risk
Pre-existing diabetes in the mother, whether type 1 or type 2, is one of the strongest known environmental risk factors for congenital heart defects. Women with pregestational diabetes face roughly an eightfold higher risk of their baby developing a heart defect compared to women without diabetes. HLHS is specifically among the defect types that occur more frequently in pregnancies complicated by diabetes.
The mechanism appears to involve how high blood sugar disrupts the early development of heart progenitor cells, the stem cells that differentiate into the various structures of the heart during the first weeks of pregnancy. Animal studies have shown that diabetic conditions alter the activity of several key cardiac development genes, including NOTCH1 and NKX2.5, the same genes independently linked to HLHS through genetic studies. This overlap suggests that diabetes may trigger HLHS through some of the same molecular pathways as inherited gene mutations.
Pesticide Exposure During Pregnancy
Maternal occupational exposure to pesticides, particularly a combination of herbicides and insecticides, has been linked to a significantly elevated risk of HLHS. Data from the National Birth Defects Prevention Study found that mothers exposed to both herbicides and insecticides during pregnancy had roughly three times the odds of having a baby with HLHS compared to unexposed mothers. At higher exposure levels, the risk climbed to about five times the baseline. The study also observed what appeared to be a dose-response relationship, meaning higher pesticide exposure correlated with higher risk.
These findings come with caveats. The number of exposed cases in the study was small, and the researchers acknowledged they could not entirely rule out chance findings from analyzing many defect types simultaneously. Still, HLHS stood out among all the heart defect subtypes as having the most consistent association with pesticide exposure, making it an area of active concern for occupational health during pregnancy.
Why Most Cases Have No Clear Explanation
Despite all the identified risk factors, the honest answer for most families is that no single cause can be pinpointed. HLHS likely results from a combination of small genetic susceptibilities and environmental triggers that converge during a narrow window of heart development in the first trimester. A baby might carry a subtle gene variant that makes the aortic valve slightly more prone to narrowing, and an additional factor, whether metabolic, environmental, or simply random variation in development, tips the balance toward incomplete growth.
This multifactorial nature is why HLHS is so difficult to predict or prevent. There is no single test that identifies at-risk pregnancies before the defect develops, and no known intervention that can reverse the process once it begins. Prenatal ultrasound can detect HLHS as early as 18 to 22 weeks, which allows families and medical teams to plan for delivery at a center equipped for immediate cardiac care, but the underlying causes are typically already set in motion long before that scan takes place.