An infrarenal abdominal aortic aneurysm is a bulging or ballooning of the body’s largest artery in the section just below where the arteries to the kidneys branch off. It’s defined as an aortic diameter of 3.0 cm or larger at that location, and it’s the most common type of aortic aneurysm. Most abdominal aortic aneurysms begin in this infrarenal segment, and about half also extend into the iliac arteries that carry blood down into the legs.
Why “Infrarenal” Matters
The aorta runs from the heart down through the chest and abdomen before splitting into two branches at the pelvis. Along the way, it sends arteries to the kidneys (renal arteries). “Infrarenal” simply means below those kidney arteries. This distinction isn’t just anatomical trivia. It tells doctors exactly where the weak spot is, which directly affects how the aneurysm is monitored and whether it can be repaired with less invasive techniques.
Some aneurysms sit higher, at the level of the kidney arteries (pararenal) or above them (suprarenal). Those are harder to treat because any repair has to work around the blood supply to the kidneys. The infrarenal location, while still serious, gives surgeons more options.
Why This Part of the Aorta Is Vulnerable
The infrarenal aorta is structurally different from the aorta higher up in the body. It contains fewer layers of elastic tissue and a lower ratio of elastin to collagen compared to the thoracic (chest) aorta. Elastin is what lets the artery stretch and snap back with each heartbeat. With less of it, this section of the aorta is more susceptible to weakening over time.
As the elastic fibers degrade, the artery wall loses its ability to recoil. The load shifts to stiffer collagen fibers at smaller stretches, which raises the peak stress on the wall for any given blood pressure. Smooth muscle cells in the wall also die off or change behavior, reducing the body’s ability to repair the damage. The result is a self-reinforcing cycle: the wall weakens, the artery expands, and the expansion increases wall stress further, driving continued growth.
Who Gets Them
Infrarenal AAAs are far more common in men than women. Among people over 65, prevalence ranges from 1.7% to 4.5% in men compared to 0.5% to 1.3% in women. That three-to-four-fold difference is one reason screening guidelines focus heavily on men. However, the gap may be partly an artifact of how aneurysms are measured. The standard 3.0 cm cutoff doesn’t account for the fact that women naturally have smaller aortas. When researchers use a ratio-based definition (comparing the infrarenal diameter to the normal segment above it), AAA prevalence in women jumps to as high as 6% to 10%.
The strongest risk factors are smoking history, older age, male sex, and family history. Smoking is the dominant modifiable risk. Even among women over 70 who actively smoke, prevalence reaches about 2.1%.
How It’s Found
Most infrarenal AAAs cause no symptoms until they’re large or rupture. They’re typically discovered during imaging done for another reason or through screening. Abdominal ultrasound is the standard screening tool because it’s painless, widely available, and involves no radiation. It’s highly specific, meaning it rarely flags an aneurysm that isn’t there. However, its sensitivity is moderate, catching roughly 57% to 70% of aneurysms in validation studies.
CT scanning without contrast performs better, detecting 83% to 89% of aneurysms and picking up cases that ultrasound misses. CT angiography (with contrast dye) provides the most detailed images and is the go-to test when planning for repair, since surgeons need precise measurements of the aneurysm’s shape, length, and relationship to surrounding arteries.
Screening Recommendations
The U.S. Preventive Services Task Force recommends a one-time ultrasound screening for men aged 65 to 75 who have ever smoked. This carries a “B” recommendation, meaning there’s moderate certainty of meaningful benefit. For men in that age range who have never smoked, selective screening is recommended based on individual risk factors like family history.
For women, the picture is less clear. The Task Force recommends against routine screening in women who have never smoked and have no family history, because the harms (anxiety, unnecessary follow-up, and potential overtreatment) outweigh the benefits in that group. For women aged 65 to 75 who have smoked or have a family history, the evidence is considered insufficient to make a firm recommendation either way.
Size, Growth, and Rupture Risk
Aneurysm diameter is the single most important factor in determining risk. Small aneurysms (under about 4 cm) grow slowly and rarely rupture, so they’re monitored with periodic imaging rather than treated. As the diameter increases, rupture risk accelerates. Aneurysms between 4.0 and 5.4 cm carry a meaningfully different rupture rate than those above 5.5 cm, and the risk climbs steeply beyond 6 cm.
Growth rate also matters. Most small AAAs expand by 1 to 3 mm per year, but any aneurysm growing faster than about 5 mm in six months gets closer attention regardless of its current size.
When Repair Is Considered
The widely accepted thresholds for elective repair are 5.5 cm for men and 5.0 cm for women. The lower threshold for women reflects both their smaller baseline aortic size and evidence suggesting their aneurysms rupture at smaller diameters.
These thresholds are under active discussion. A modeling study published in the Journal of Vascular Surgery found that for a 60-year-old man in average health, the size that best minimized aneurysm-related death was actually 6.9 cm, and for a 60-year-old woman, 6.1 cm. Life expectancy varied by less than two months across a fairly wide range of repair sizes. Patients in poor overall health had even higher optimal thresholds, above 6.5 cm for women and 7.0 cm for men. The takeaway is that the decision to repair involves more than diameter alone: age, overall health, and aneurysm growth rate all factor in.
Endovascular Repair
Endovascular aneurysm repair, commonly called EVAR, is a minimally invasive procedure. A fabric-covered stent is threaded up through arteries in the groin and positioned inside the aneurysm. Once deployed, it creates a new channel for blood flow, sealing off the weakened bulge so blood pressure no longer pushes against the damaged wall.
Not every infrarenal aneurysm is a good candidate for EVAR. The “neck” of the aneurysm, the stretch of normal aorta between the kidney arteries and the top of the bulge, needs to be long enough and healthy enough for the stent to anchor securely. Ideally, this neck is at least 1.5 cm long, relatively straight, and free of heavy calcium deposits or blood clot. A short neck (under 1.0 cm), a sharp angle, or significant disease in the neck wall make EVAR riskier and may rule it out.
Recovery after EVAR is relatively quick. The average hospital stay is about 2 to 3 days, and many patients go home within two days. Those who stay longer than three days tend to have higher rates of complications, often related to heart or kidney function. EVAR does require lifelong follow-up imaging, typically with CT scans, to check for issues like blood leaking around the stent (endoleak) or the graft shifting position.
Open Surgical Repair
Open repair involves a large abdominal incision. The surgeon clamps the aorta above and below the aneurysm, opens the bulge, and sews in a synthetic graft to replace the damaged segment. It’s a major operation with a longer recovery, typically requiring several days in the hospital and weeks to months before returning to full activity.
The tradeoff is durability. A study using matched Medicare data found that at six years, open repair was associated with a 17% lower risk of death, a 24% lower risk of late rupture, and a 33% lower risk of needing a second procedure compared to EVAR. Reintervention rates at six years were 11.6% for open repair versus 16.0% for EVAR. Open repair essentially solves the problem once, while EVAR requires ongoing surveillance because the stent can develop complications over time.
For younger, healthier patients who can tolerate a bigger operation, open repair often makes sense because they’ll live long enough for EVAR’s long-term disadvantages to matter. For older patients or those with significant health problems, EVAR’s lower upfront stress on the body can be the better choice.
Living With a Small Aneurysm
If your aneurysm is below the repair threshold, your doctor will set up a surveillance schedule with regular ultrasounds or CT scans to track its size. The interval depends on the diameter: very small aneurysms (3.0 to 3.9 cm) might be checked every two to three years, while those approaching 5 cm are typically imaged every six months.
Quitting smoking is the single most impactful thing you can do to slow growth. Managing blood pressure reduces the mechanical stress on the weakened wall. Staying physically active is generally safe and encouraged, though your doctor may advise avoiding heavy straining, like maximal-effort weightlifting, that spikes blood pressure sharply. Most people with small, monitored aneurysms live normal lives for years without ever needing repair.