Congenital heart defects are the most common type of birth defect, affecting roughly 3 out of every 1,000 babies born worldwide. About 40% of cases trace back to genetic factors, around 5% to environmental exposures, and the remaining 55% have no identifiable cause. That large unknown category can feel frustrating if you’re looking for answers, but understanding the known causes and risk factors can still help you make sense of what happened or take steps to lower your risk in a future pregnancy.
The Critical Window for Heart Development
The fetal heart begins forming very early, with most of its major structures taking shape between weeks 3 and 8 of pregnancy. During this period, a simple tube loops, folds, and divides into four chambers with valves and separate outflow vessels. The most vulnerable phase appears to be when the heart is dividing into chambers and forming its outlet structures. Animal studies have identified a narrow window, roughly corresponding to the late first trimester in humans, when disruptions like oxygen deprivation can cause structural defects. After that window closes, the same disruptions no longer produce heart malformations.
This timing matters because many women don’t realize they’re pregnant until several weeks in. By then, the heart’s basic architecture is already being built, which is why preconception health plays such an outsized role.
Genetic and Chromosomal Causes
Chromosomal abnormalities account for an estimated 9% to 18% of all congenital heart defects. Down syndrome (trisomy 21) is the most common genetic diagnosis found alongside a heart defect. Trisomy 18, trisomy 13, and Turner syndrome (monosomy X) make up most of the remaining chromosomal causes. Children with these conditions frequently have holes between heart chambers, valve abnormalities, or complex defects involving multiple structures.
Beyond whole-chromosome conditions, smaller genetic deletions also play a significant role. 22q11.2 deletion syndrome (sometimes called DiGeorge syndrome) is one of the most important. It accounts for a meaningful share of heart defects, particularly those affecting the outflow tracts, the vessels that carry blood out of the heart to the lungs and body. These defects can occur even when the deletion is too small to cause obvious facial features or other signs.
Heart defects can also result from single-gene mutations that don’t show up on standard chromosome tests. Some run in families, while others arise spontaneously. If you or your partner have a congenital heart defect or a family history of one, the risk to your baby is higher than the general population’s, though still relatively small in absolute terms.
Maternal Diabetes
Preexisting diabetes is one of the strongest modifiable risk factors. Women with diabetes diagnosed before pregnancy have roughly four times the baseline risk of having a baby with a heart defect: about 318 per 10,000 births compared to 80 per 10,000 in the general population, based on a large nationwide cohort study published in Circulation. Women who had experienced serious diabetes complications before pregnancy faced an even higher risk, around 7.6 times the baseline.
The mechanism centers on high blood sugar during the earliest weeks of pregnancy. Excess glucose doesn’t damage DNA directly, but it disrupts the signaling pathways that guide heart cells as they migrate and organize. It can also trigger epigenetic changes, essentially flipping certain genes on or off at the wrong time. The types of defects most strongly linked to maternal diabetes involve the outflow tracts, the structures connecting the heart to the major arteries. This pattern aligns with lab research showing that specific cell populations in the developing heart’s outflow region are especially sensitive to high glucose.
Gestational diabetes, which develops later in pregnancy, carries a much smaller risk because the heart’s critical formation period has typically passed by the time blood sugar rises.
Infections During the First Trimester
Certain infections early in pregnancy increase the odds of heart defects. Rubella has the strongest association, nearly tripling the risk. Though rubella is now rare in countries with widespread vaccination, it remains a concern in parts of the world with lower immunization coverage. Coxsackievirus, respiratory infections, and influenza during the first trimester are each associated with a roughly 50% to 57% increase in risk.
A meta-analysis in the European Heart Journal found that first-trimester infections of any kind were most closely tied to septal defects, the holes between heart chambers that are the most common type of congenital heart malformation. The risk appears concentrated in the first trimester, aligning with the heart’s vulnerable developmental window.
Medications and Substances
Several classes of medication can interfere with fetal heart development when taken during early pregnancy. Blood pressure medications are among the most studied. ACE inhibitors and related drugs (angiotensin receptor blockers) have been linked to several types of heart defects, with risk increases ranging from roughly three to six times higher depending on the specific defect. Beta-blockers carry a similar pattern of elevated risk for valve abnormalities and septal defects.
Other medications with known links to heart defects include certain anti-seizure drugs, lithium, and high-dose vitamin A derivatives (retinoids) used for severe acne. The risk depends heavily on timing and dosage. If you’re taking any of these medications and planning a pregnancy, working with your doctor before conception to adjust your treatment is the single most effective step you can take.
Maternal smoking is also associated with a modest increase in heart defect risk, though the effect is smaller than that of diabetes or chromosomal conditions.
Obesity and Body Weight
Women who are overweight or obese before pregnancy face about a 26% higher risk of having a baby with a heart defect compared to women at a normal weight. The relationship follows a dose-response pattern, meaning the risk climbs as BMI increases. The biological explanation likely overlaps with the diabetes pathway: higher body weight is associated with insulin resistance and metabolic changes that can affect early fetal development, even in women who don’t meet the threshold for a diabetes diagnosis.
Assisted Reproductive Technology
Babies conceived through IVF or similar assisted reproduction have a modestly higher rate of heart defects compared to naturally conceived babies. A large Nordic study published in the European Heart Journal found major heart defects in 1.84% of children born after assisted reproduction versus 1.15% of naturally conceived children, an adjusted increase of about 36%. For severe heart defects specifically, the increase was around 30%.
When researchers looked only at singleton pregnancies (removing the effect of twins and triplets, which are more common with fertility treatment), the gap narrowed but remained statistically significant: about a 19% higher risk for major defects. Interestingly, there was no meaningful difference between standard IVF and ICSI (where a single sperm is injected directly into the egg), or between fresh and frozen embryo transfers. This suggests the increased risk may relate more to the underlying fertility issues or hormonal treatments than to the specific lab technique used.
Paternal Age
Advanced paternal age has gotten attention as a potential risk factor, but the data is largely reassuring. A national cohort study found no overall association between the father’s age and the baby’s risk of heart defects after accounting for maternal age and other factors. The one exception was a specific subtype called patent ductus arteriosus, where fathers over 45 had a 69% higher risk compared to fathers aged 25 to 29. For all other major types of heart defects, including septal defects and complex conditions, paternal age did not appear to matter.
When Multiple Factors Overlap
In most cases, a congenital heart defect results from an interaction between genetic susceptibility and environmental exposures rather than a single clear-cut cause. A fetus with a mild genetic predisposition might develop normally unless it’s also exposed to high blood sugar, a medication, or an infection during the critical window. This “multi-hit” model helps explain why the same exposure causes a defect in one pregnancy but not another, and why more than half of all cases remain unexplained even after thorough evaluation.
For parents who’ve had a child with a heart defect and want to understand their risk in future pregnancies, genetic counseling can clarify whether a chromosomal or single-gene cause was involved. If no genetic cause is found, the recurrence risk for most common heart defects is in the range of 2% to 6%, higher than the general population but still meaning the large majority of subsequent pregnancies are unaffected.