The prostate gland is a small, walnut-sized structure found in the male reproductive system, positioned directly below the bladder. It completely encircles the urethra, the tube that transports urine and semen out of the body. The primary role of the prostate is to produce the fluid component of semen, which nourishes and transports sperm during ejaculation. Because of its location surrounding the urethra, any change in size directly impacts a man’s ability to urinate, making its dimensions a significant health concern.
Defining Normal Prostate Dimensions
The typical size of a healthy prostate in a young adult is often compared to a walnut. Clinicians define the “normal” volume for men under 40 as being between 20 and 30 cubic centimeters (cc). This volume is roughly equivalent to a weight of 20 to 25 grams.
Physical dimensions in this normal range are approximately 4 centimeters wide, 3 centimeters in length (or height), and 2 centimeters in depth. These values serve as a baseline for assessment, though size can be influenced by genetics and overall health. The prostate has a natural tendency to grow throughout a man’s life, and this age-related increase is considered physiological, not pathological.
Methods Used to Measure Prostate Size
Clinicians use several methods to determine prostate size, with varying degrees of accuracy. The most common initial assessment is the Digital Rectal Exam (DRE), where a provider manually estimates the size and consistency of the gland by feeling its posterior surface through the rectum. While the DRE is quick and detects abnormalities, it is a subjective technique that often underestimates the true volume, especially for larger prostates.
For a more precise measurement of volume, Transrectal Ultrasound (TRUS) is utilized. This procedure involves inserting a small ultrasound probe into the rectum to create detailed images of the prostate. The volume is then calculated using the prolate ellipsoid formula, which multiplies the gland’s length, width, and height by a correction factor of 0.52.
Magnetic Resonance Imaging (MRI) provides highly accurate and detailed prostate measurements. Although MRI is more expensive and not routinely used for size assessment alone, some studies suggest it may be the most accurate predictor of actual volume compared to surgical specimens. A volume greater than 30 cc is the common clinical threshold used to define an enlarged prostate.
The Primary Causes of Prostate Enlargement
The most frequent, non-cancerous reason for the prostate to grow larger is Benign Prostatic Hyperplasia (BPH). BPH is strongly linked to the aging process, with prevalence increasing significantly after age 50. By age 85, nearly 90% of men will have some degree of BPH.
The growth is driven by changes in the balance of sex hormones over time. Specifically, an increase in dihydrotestosterone (DHT), a potent derivative of testosterone, stimulates the proliferation of prostate cells. This cellular growth usually occurs in the transitional zone, the area immediately surrounding the urethra. BPH is a benign condition, meaning it is not malignant and does not increase the risk of prostate cancer.
How Size Affects Urinary Function
The physical enlargement of the prostate directly affects urinary function because of its unique anatomical position. As the gland grows, it crowds and compresses the urethra, creating an obstruction to the flow of urine out of the bladder. This mechanical blockage is known as the static component of the obstruction.
The resulting symptoms are collectively referred to as Lower Urinary Tract Symptoms (LUTS). These symptoms are categorized as either voiding (obstructive) or storage (irritative). Voiding symptoms include a weak or intermittent urine stream, the need to strain to start urination (hesitancy), and the sensation of incomplete bladder emptying.
Storage symptoms result from the bladder muscle working harder against the obstruction, leading to changes in bladder function. These symptoms include urinary frequency, a sudden and compelling need to urinate (urgency), and nocturia (the need to wake up one or more times during the night to pass urine). Though enlargement degree does not always correlate perfectly with symptom severity, a prostate exceeding 30 cc is more likely to cause moderate to severe LUTS.