When a baby is born with a heart that didn't form quite right, parents are left with a heavy question: Why? Congenital heart defects (CHDs) are the most common type of birth defect, affecting nearly 1 in 100 newborns worldwide. Yet, despite decades of research, the exact cause often remains unknown. Here, cardiologists and researchers explain what they have learned so far about the origins of these structural heart problems, and what that means for families looking for answers.
How do congenital heart defects develop?
To understand the causes, it helps to know the timeline. A baby’s heart begins forming in the first few weeks of pregnancy—often before a person even knows they are pregnant. By week eight, the four chambers, valves, and major blood vessels are mostly formed. Anything that disrupts this precise, rapid process can lead to a structural defect.
“Think of it like building a house in a very short amount of time,” says Dr. Amita Bhardwaj, a pediatric cardiologist. “If the builder runs out of supplies, gets bad materials, or is interrupted, the walls might not finish straight. That is essentially what happens in the developing heart.”
A quick note: Most babies born with a CHD have no identifiable risk factors. These are not typically linked to anything a parent did or didn't do.
What specific factors are known to play a role?
While many cases remain unexplained, cardiologists point to a combination of genetic and environmental influences. Here is what the evidence shows so far.
Genetic mutations and inherited conditions
Approximately 15 to 20 percent of CHDs can be traced to a known genetic cause. Some cases involve a single gene mutation, while others are linked to missing or extra chromosomes. Conditions such as Down syndrome (trisomy 21), Turner syndrome, and DiGeorge syndrome have a well-established association with heart defects.
Researchers have also identified specific genes—like NKX2-5 and GATA4—that regulate early heart development. When these genes mutate, the heart may not form its chambers, valves, or septa properly. New parents are often surprised to learn that some of these genetic changes run in families and may have caused no obvious problems in the parents themselves.
Environmental exposures and maternal health
Several external factors during pregnancy can increase the risk of a CHD, though they rarely guarantee one will occur. These include:
- Infections such as rubella (German measles) during early pregnancy
- Uncontrolled diabetes—both type 1 and gestational diabetes have been linked to higher CHD rates
- Certain medications like some antiseizure drugs and isotretinoin (a former acne treatment)
- Alcohol consumption during early pregnancy, which disrupts fetal cell migration
- Maternal obesity, which is associated with mild increases in certain defects
Importantly, exposure to a known risk factor does not mean a defect will occur. It simply raises the odds from very low to slightly higher, and many babies exposed to the same factors are born with normal hearts.
Folic acid and nutrition
One of the clearest “risk reducers” that cardiologists emphasize is folic acid. Studies have shown that women who take a daily folic acid supplement (400 to 800 micrograms) before and during early pregnancy reduce the risk of both neural tube defects and some heart defects. The protective effect is modest but real, and it is one of the few modifiable factors a patient can control before conception.
Is there a single cause, or many?
For most families, the answer is: many. Current research strongly supports a “multifactorial” model. In this model, a baby inherits a slight genetic vulnerability from one or both parents. Then, an environmental trigger (or a random developmental error) tips the balance toward a structural defect. Because the heart forms in a narrow window, even a small genetic variation can become significant when combined with an external factor.
“It is rarely one thing,” says Dr. Robert Chen, a cardiologist specializing in adult congenital heart disease. “Most cases appear to be a perfect storm of small factors. That can be frustrating for parents who want a single explanation, but it also means there is usually no one to blame.”
Can congenital heart defects be prevented?
Because most causes are either genetic or random, full prevention is not currently possible. That said, cardiologists recommend several steps to lower risk as much as possible:
- Achieve good blood sugar control before pregnancy if you have diabetes
- Take a daily folic acid supplement starting at least one month before conception
- Avoid alcohol entirely during the first trimester
- Ensure vaccinations (especially rubella) are up to date before pregnancy
- Discuss any prescription medications with your healthcare provider if you are planning pregnancy
During pregnancy, detailed ultrasounds between 18 and 22 weeks can often detect major heart defects before birth. Early diagnosis allows families and doctors to plan for immediate care after delivery—including, if needed, a transfer to a hospital with specialized pediatric cardiology and cardiac surgery services.
What is the outlook for children born with a CHD?
Survival and quality of life for children with CHDs have improved dramatically over the past 50 years. According to the American Heart Association, more than 90 percent of children born with a CHD now survive to adulthood—including those with complex, single-ventricle hearts. Many lead active lives, attend school, play sports, and eventually have families of their own.
Adults living with CHD require lifelong cardiology follow-up, often at a specialized adult congenital heart disease (ACHD) center. As surgical techniques and medical management continue to improve, even children born with severely underdeveloped hearts can achieve full, meaningful lives.
If you or a loved one has received a diagnosis of congenital heart disease, know that you are not alone. Talk with a pediatric or adult CHD specialist about your specific type of defect and what monitoring or interventions may be appropriate over time.
