Congenital heart disease can be diagnosed before birth, within hours of delivery, or years later in adulthood, depending on the type and severity of the defect. The diagnostic process typically starts with noninvasive screening and imaging, then moves to more detailed tests if needed. Some defects are caught during a routine pregnancy ultrasound, others by a simple oxygen sensor clipped to a newborn’s foot, and some go undetected until symptoms appear in childhood or adulthood.
Prenatal Detection During Pregnancy
The first opportunity to diagnose a heart defect comes during the routine second-trimester ultrasound, when the sonographer checks the baby’s heart chambers and blood flow as part of a standard anatomy scan. If anything looks abnormal, or if the pregnancy carries higher risk factors, a specialized fetal echocardiogram is ordered. This is a detailed ultrasound focused entirely on the baby’s heart, and the optimal window is between 18 and 22 weeks of gestation.
Early fetal echocardiography, performed before 16 weeks, is reserved for pregnancies at the highest risk. That includes cases where the fetus showed increased fluid behind the neck (nuchal translucency of 3.5 mm or more) on first-trimester screening, a previously affected sibling, a known chromosomal abnormality, or identical twins sharing a placenta.
Several maternal conditions also prompt a fetal echocardiogram: diabetes (including gestational diabetes), autoimmune conditions like lupus, use of assisted reproductive technology, certain infections during pregnancy such as rubella or parvovirus, and exposure to medications known to affect fetal development. A family history of congenital heart disease in a first-degree relative of the fetus is another common reason.
Newborn Pulse Oximetry Screening
Every state in the U.S. screens newborns for critical congenital heart disease using pulse oximetry, a painless test that measures oxygen levels in the blood through a small sensor on the skin. The test is performed when the baby is at least 24 hours old, awake, and breathing on their own without supplemental oxygen.
Sensors are placed on the right hand and one foot. A passing result requires oxygen saturation of 95% or higher in both locations, with no more than a 3% difference between the two readings. The screen is considered failed if any reading drops below 90%, or if saturation stays below 95% or the difference between hand and foot exceeds 3% on two separate measurements taken an hour apart.
This screening catches defects that reduce oxygen levels in the blood, such as certain problems with the heart’s valves, chambers, or major blood vessels. It is not a perfect test. Sensitivity ranges from 50% to 76%, meaning it misses some types of critical heart defects, particularly those that don’t cause low oxygen in the first day or two of life. A normal pulse oximetry result does not rule out congenital heart disease entirely, which is why physical exams and other follow-up remain important.
Physical Exam and Early Signs
Doctors listen for heart murmurs during routine newborn and well-child exams. A murmur is an extra or unusual sound caused by turbulent blood flow, and while many murmurs are harmless, certain patterns point to structural problems like holes between heart chambers or narrowed valves. Visible signs can include a bluish tint to the lips or fingernails, which signals that not enough oxygen is reaching the body. In infants, rapid breathing, poor feeding, and slow weight gain can also raise suspicion.
Some defects produce subtler clues. A difference in pulse strength between the arms and legs, for instance, can suggest a narrowing of the aorta. These findings on physical exam typically lead to further testing rather than a definitive diagnosis on their own.
Echocardiography: The Primary Diagnostic Tool
The echocardiogram is the cornerstone of congenital heart disease diagnosis at every age. It uses ultrasound waves to create real-time images of the heart’s chambers, walls, valves, and blood vessels. The test is painless, uses no radiation, and can be performed at a bedside or in an outpatient clinic.
Two-dimensional imaging shows the physical structure of the heart: the size of chambers, the position of walls, whether holes exist between the upper or lower chambers, and whether valves open and close properly. A hole in the wall between the upper chambers (atrial septal defect) appears differently depending on its location. The most common type sits in the center of the wall, while others occur near the bottom or near the junction with the large veins entering the heart. Each type has different implications for treatment.
Doppler imaging adds another layer by tracking blood flow. Color-coded overlays show the direction and speed of blood moving through the heart. This is especially valuable for detecting shunts, where blood flows through an abnormal opening from one side of the heart to the other, or for identifying a vessel that should have closed after birth but remains open. In a patent ductus arteriosus, for example, Doppler reveals a jet of blood flowing from the aorta into the pulmonary artery through a persistent connection. Doppler also quantifies how severe a narrowing is by measuring the pressure difference across a valve or vessel.
EKG Patterns That Point to Specific Defects
An electrocardiogram (EKG) records the heart’s electrical activity and can reveal patterns characteristic of certain defects. It doesn’t diagnose a structural problem directly, but specific abnormalities on the tracing help narrow down what’s going on and guide further imaging.
A particular type of atrial septal defect located at the bottom of the wall between the upper chambers consistently produces an abnormal leftward electrical axis, a signature that helps distinguish it from other types. The more common central type of atrial septal defect often shows an incomplete block in the right side of the heart’s electrical wiring, and a distinctive notch in certain waveforms that correlates with the size of the hole. In tetralogy of Fallot, one of the more common cyanotic heart defects, the EKG shows thickening of the right ventricle because it has been working harder than normal against an obstruction. Widening of the electrical signal over time in patients with repaired tetralogy of Fallot is monitored closely because it can signal increasing risk of dangerous heart rhythm problems.
Cardiac MRI and CT for Complex Cases
When echocardiography doesn’t provide a complete picture, cardiac MRI and CT fill in the gaps. Each has distinct strengths.
Cardiac MRI is considered the gold standard for measuring the size and pumping function of the heart’s chambers, and it produces highly accurate flow measurements. Unlike echocardiography, it isn’t limited by body size or the available angles for imaging. The heart can be examined from any direction, making it particularly useful for complex defects involving multiple abnormalities. It also uses no radiation, which matters for patients who need repeated imaging over a lifetime.
Cardiac CT excels at visualizing very small structures, especially blood vessels and coronary arteries, because of its high spatial resolution. In newborns, CT is often used to map out blood vessel anatomy that echocardiography can’t fully capture. For conditions like pulmonary atresia with multiple abnormal blood vessels supplying the lungs, CT provides the detailed vascular roadmap that surgeons need. CT is also the preferred choice after stent or device placement, since metal implants create interference on MRI that can make images unreadable. It’s better at detecting calcium buildup in and around the heart, which matters when planning certain procedures.
Cardiac Catheterization
Cardiac catheterization involves threading a thin, flexible tube through a blood vessel into the heart to directly measure pressures inside each chamber and the blood vessels connected to it. This provides information that no external imaging test can match. Pressures in the right side of the heart, the pulmonary arteries, and the left side of the heart are recorded individually, and blood samples taken during the procedure measure oxygen levels at each location.
These measurements allow doctors to calculate how much blood is flowing through the heart per minute, how much resistance the lung blood vessels are putting up, and how much blood is crossing abnormal connections. This level of detail is essential when deciding whether a patient is a good candidate for surgery or a catheter-based repair, particularly in more complex defects where imaging alone doesn’t tell the full story.
Genetic Testing
Genetic evaluation is now a standard part of the workup for many patients with congenital heart disease. Chromosomal abnormalities account for 8% to 12% of all cases, with Down syndrome (trisomy 21) being the most common. Other chromosomal conditions frequently linked to heart defects include trisomy 18 and Turner syndrome.
Beyond full chromosomal abnormalities, smaller genetic deletions and duplications also cause heart defects. The 22q11.2 deletion syndrome (sometimes called DiGeorge syndrome) is the most common of these, often associated with specific outflow tract defects. Chromosomal microarray testing can detect these smaller changes that a standard chromosome analysis would miss.
The American College of Medical Genetics and Genomics now recommends exome or genome sequencing as first-line testing for individuals with congenital anomalies like heart defects. These broader tests analyze thousands of genes at once and can identify causes that targeted gene panels might miss. Genetic results matter not just for understanding the heart defect itself but for predicting other health issues that may be part of the same syndrome, and for informing family planning in future pregnancies.
Diagnosis in Adults
Not all congenital heart disease is caught in childhood. Some defects, particularly smaller holes between chambers or mildly abnormal valves, may cause no symptoms for decades. In a study at a tertiary cardiac hospital, ventricular septal defects and tetralogy of Fallot were the most frequently diagnosed conditions in adults, accounting for about 30% and 25% of cases respectively. Delayed referral was the most common reason for late diagnosis (43% of cases), followed by patients putting off seeking care and initial misdiagnosis by providers.
Adults are typically diagnosed after developing symptoms like unexplained shortness of breath, exercise intolerance, irregular heartbeats, or swelling. A heart murmur heard during a routine physical may also trigger investigation. The diagnostic workup follows the same sequence as in children: echocardiography first, then MRI or CT for more detail, catheterization if pressure measurements are needed, and genetic testing when appropriate.