Sciatic nerve pain happens when something presses on, irritates, or inflames the longest nerve in your body. This nerve runs from your lower back through your hips and buttocks, then down each leg. The lifetime incidence of sciatica is estimated between 13% and 40%, making it one of the most common pain conditions people experience. The causes range from structural problems in your spine to muscle issues and even chemical irritation that can trigger pain without any physical compression at all.
Herniated Discs: The Most Common Cause
A herniated disc in the lower back is the leading reason people develop sciatica. Between each vertebra in your spine sit cushioning discs with a tough outer shell and a soft, gel-like center. When that outer shell tears (from wear, injury, or repetitive stress), the inner material pushes outward. If this bulge presses against a nerve root where the sciatic nerve originates, it triggers pain that can radiate all the way down your leg.
But the disc material doesn’t always have to physically touch the nerve to cause problems. When the inner disc material leaks out, it releases inflammatory chemicals into the surrounding space. These chemicals can damage the protective coating around nerve fibers, creating hypersensitive spots that fire off pain signals on their own. This is why some people have severe sciatica even when imaging shows only a small disc bulge that doesn’t appear to be pressing on anything. The inflammation itself is enough to irritate nearby nerve roots and produce radiating leg pain.
Spinal Stenosis and Aging
Spinal stenosis occurs when the tunnel running through your vertebrae gradually narrows, leaving less room for the spinal cord and the nerve roots branching off it. As the space shrinks, nerves get cramped, compressed, and irritated. Degenerative spinal changes affect up to 95% of people by age 50, and stenosis is one of the more common results of that process.
The narrowing typically develops over years as a combination of factors pile up: thickened ligaments, bone spurs, and worn-down discs all contribute to a tighter spinal canal. Pain from stenosis tends to come on gradually rather than suddenly, and it often worsens when you stand or walk for extended periods. Sitting or leaning forward may provide temporary relief because those positions slightly open the spinal canal.
Spondylolisthesis: When a Vertebra Slips
Spondylolisthesis is a condition where one vertebra slides forward over the one beneath it, putting pressure on the surrounding nerves. Doctors grade the slippage on a scale from I to IV. Most cases fall into the low-grade category (Grade I or II), which are more manageable and less likely to need surgery. High-grade cases (Grade III and IV) compress nerves more severely and are much more likely to require surgical intervention to relieve pressure on the sciatic nerve.
This condition can develop from a stress fracture in the vertebra (common in young athletes who do repetitive back extension, like gymnasts) or from age-related degeneration. The L5-S1 level at the base of your spine is the most frequently affected spot, which is right where the sciatic nerve roots exit.
Piriformis Syndrome
Not all sciatic pain starts in the spine. The piriformis is a small muscle deep in your buttock that sits directly on top of the sciatic nerve. When this muscle becomes inflamed, goes into spasm, or develops scar tissue, it can compress the nerve and mimic the symptoms of spine-related sciatica.
Some people are also born with anatomical variations where the sciatic nerve actually passes through the piriformis muscle rather than beneath it, making them more vulnerable to compression. That said, piriformis syndrome is relatively uncommon. It accounts for roughly 0.3% to 6% of all lower back and leg pain cases. It’s often considered a diagnosis of exclusion, meaning doctors typically rule out spinal causes first.
Pregnancy and Hormonal Changes
Pregnancy creates a perfect storm of factors that can irritate the sciatic nerve. As the uterus grows, it shifts your center of gravity forward and increases the load on your lower spine. The baby’s position can also put direct pressure on the nerve, particularly in the later months.
Hormonal changes add another layer. Your body produces a hormone called relaxin that loosens muscles, ligaments, and joints so your body can stretch to accommodate the pregnancy. Relaxin levels peak around 12 to 14 weeks, loosening the ligaments around your pelvis and lower back. While this flexibility is essential for delivery, it can destabilize your spine and alter your posture. The combination of a looser, less stable lower back and the increasing weight of pregnancy can compress or irritate the sciatic nerve. Many pregnant people also experience pelvic girdle pain across one or both sides of the lower back as a related effect of these changes.
How Chemical Irritation Works Without Compression
One of the less intuitive causes of sciatica is pure chemical irritation. You can have significant nerve pain even when nothing is physically pressing on the nerve. Research has shown that damaged or degenerating discs produce inflammatory substances that can leak through tears in the disc wall and seep into the space surrounding the nerve roots. These chemicals break down the nerve’s protective insulation, creating regions that become hypersensitive and fire off pain signals spontaneously.
This helps explain a frustrating experience many sciatica patients have: imaging that looks relatively normal despite real, significant pain. The nerve damage in these cases is chemical rather than mechanical, and it doesn’t always show up clearly on an MRI. It also explains why anti-inflammatory treatments can be effective for sciatica even when there’s no obvious structural problem compressing the nerve.
What Sciatica Feels Like
Sciatic nerve pain typically follows a distinct pattern. It usually affects only one side of the body. The pain can range from a dull ache to a sharp, burning sensation or even an electric shock-like jolt that shoots down the back of your leg. Numbness, tingling, or muscle weakness in the affected leg are also common. Coughing, sneezing, or sitting for long periods often makes it worse because these actions increase pressure on the nerve.
The location of your symptoms can hint at which nerve root is involved. Pain that runs down the back of your thigh to your calf often points to a different level of compression than pain that radiates to the outside of your foot. This is useful information for your doctor when narrowing down the cause.
Emergency Warning Signs
In rare cases, severe nerve compression in the lower spine can develop into cauda equina syndrome, a medical emergency. This happens when the bundle of nerve roots at the base of the spinal cord becomes severely compressed, and it requires urgent treatment to prevent permanent damage.
The red flags include loss of sensation in the area that would touch a saddle (inner thighs, groin, buttocks), difficulty starting or controlling urination, bowel incontinence, weakness in both legs, and new sexual dysfunction. These symptoms can appear alongside typical sciatica pain but signal something far more serious. Importantly, you don’t need to have full incontinence or complete numbness for it to be an emergency. Even partial changes in bladder control or reduced sensation in the saddle area warrant immediate evaluation, ideally with an emergency MRI. Waiting for symptoms to worsen leads to poorer outcomes.