Tetralogy of Fallot (TOF) results from a combination of genetic factors, chromosomal abnormalities, and maternal health conditions that disrupt normal heart development during the first eight weeks of pregnancy. No single cause explains every case. In roughly 20 to 25 percent of cases, a specific genetic or chromosomal cause can be identified, while the rest are thought to involve complex interactions between multiple genes and environmental influences.
The Four Structural Defects
TOF involves four related heart abnormalities that stem from a single developmental error. Understanding them helps make sense of why certain genetic and environmental disruptions lead to this specific condition.
- Ventricular septal defect (VSD): A hole in the wall between the heart’s two lower chambers allows oxygen-rich and oxygen-poor blood to mix.
- Pulmonary stenosis: The valve and artery leading to the lungs are narrowed, restricting blood flow to the lungs where it would normally pick up oxygen.
- Overriding aorta: The aorta, the body’s main artery, sits directly over the hole between the ventricles instead of connecting only to the left side. It receives blood from both chambers.
- Right ventricular hypertrophy: The muscular wall of the right ventricle becomes abnormally thick because it has to pump harder against the narrowed pulmonary valve.
The fourth defect, the thickened right ventricle, is actually a consequence of the other three. It develops over time as the heart compensates for the increased workload.
What Goes Wrong During Development
All four defects trace back to a problem that occurs around weeks four through eight of pregnancy, when the heart’s outflow tract (the tube that eventually splits into the aorta and pulmonary artery) is forming. Normally, this tube rotates and divides evenly so the aorta lines up with the left ventricle and the pulmonary artery lines up with the right ventricle.
In TOF, two things go wrong simultaneously. The cushions of tissue inside the outflow tract that guide this division are underdeveloped, and the tube itself doesn’t rotate far enough. This insufficient rotation causes the dividing wall to shift forward and to the right, so the aorta ends up straddling both ventricles, the pulmonary artery is squeezed too narrow, and a gap is left in the wall between the ventricles. The severity of each defect depends on how far off the rotation and tissue growth were.
Chromosomal Abnormalities
The single most common identifiable cause of TOF is 22q11.2 deletion syndrome, a condition where a small piece of chromosome 22 is missing. It accounts for roughly 15 percent of all TOF cases. Children with this deletion often have other features beyond the heart defect, including immune system problems, feeding difficulties, and later speech or learning challenges. Genetic testing for this deletion is standard when a baby is diagnosed with TOF.
Down syndrome (trisomy 21) accounts for about 7 percent of TOF cases. Other chromosomal conditions like trisomy 18 and trisomy 13 are also associated with TOF, though they represent a smaller share. Taken together, chromosomal abnormalities explain roughly a quarter of all cases.
Gene Mutations
Even when chromosomes look normal under a microscope, changes in individual genes can cause TOF. Several have been identified.
NKX2-5 was the first gene directly linked to TOF. It produces a protein that acts as a master switch for heart development, turning on the downstream genes needed to build cardiac structures. Certain mutations in NKX2-5 prevent this protein from functioning properly, blocking the chain of signals that guide the outflow tract into its correct position.
JAG1 mutations are another established cause. JAG1 helps cells communicate with their neighbors through a signaling pathway that controls how heart tissue grows and differentiates. When only half the normal amount of JAG1 protein is produced, this signaling pathway misfires, disrupting the network of genes that shape the right side of the heart and outflow tract. JAG1 mutations are also responsible for Alagille syndrome, which often includes heart defects alongside liver problems.
FOXC2 and related genes in what researchers call the “second heart field” network are also implicated. These genes specifically influence the growth of right-sided heart structures and the outflow tract, which is precisely where TOF originates. Rare variants in these genes reduce their ability to activate the developmental programs needed for normal formation of the pulmonary valve and the septum between the ventricles.
In many families, no single gene mutation is found. Current thinking is that TOF often results from the combined effect of several common gene variants, each contributing a small amount of risk that adds up when enough of them occur together.
Maternal Health and Diabetes
Poorly controlled blood sugar during pregnancy is one of the strongest modifiable risk factors for TOF. A study published through the American Heart Association found that elevated maternal glucose levels were associated with a dramatically higher risk of TOF in offspring, with an adjusted odds ratio of 18.45. To put that in practical terms, the risk was roughly 18 times higher compared to pregnancies with normal glucose levels. This association was far stronger for TOF specifically than for other types of heart defects studied, suggesting something about high glucose is particularly disruptive to outflow tract development.
This applies to both pre-existing diabetes and gestational diabetes that develops early in pregnancy. The critical window is the first trimester, when the heart is actively forming. Blood sugar control before and during early pregnancy is one of the few actionable ways to reduce risk.
Maternal Age and Other Risk Factors
Maternal age over 40 is recognized as an independent risk factor for TOF. The reasons likely overlap with the increased rate of chromosomal abnormalities in pregnancies at older ages, though age-related changes in the uterine environment may also play a role.
Certain medications taken during the first trimester have been linked to outflow tract defects. Retinoic acid, used in some acne and skin treatments, is a well-established cardiac teratogen in animal studies and is strictly contraindicated during pregnancy. Other medications under investigation include certain anti-seizure drugs and lithium, though the evidence varies in strength. The relationship between alcohol consumption during pregnancy and TOF specifically has been studied extensively, but large epidemiologic studies have not shown a statistically significant increased risk for most heart defects examined, including TOF.
Folic Acid and Risk Reduction
Taking folic acid supplements during the first trimester significantly lowers the risk of TOF. A large case-control study found that first-trimester folic acid supplementation was associated with a 79 percent reduction in TOF risk, one of the strongest protective effects seen for any type of congenital heart defect. Interestingly, taking a general multivitamin without additional folic acid did not show the same benefit.
This protective effect is thought to work because folic acid is essential for DNA synthesis and cell division during the rapid growth phase when the heart’s outflow tract is forming. Deficiencies during this narrow window can impair the precise tissue growth and rotation needed for normal separation of the aorta and pulmonary artery. For anyone who missed taking folic acid before conception, starting supplementation as early as possible in the first trimester still provides meaningful protection.