Why do my testicular ultrasound measurements differ (3.1 × 3.4 cm vs ~4 cm) and my testicles feel smaller, despite a normal semen analysis?

Testicular Ultrasound Measurement Discrepancy

Your Measurements Are Likely Due to Technical Error, Not True Atrophy

The discrepancy between 3.1–3.4 cm and 4 cm testicular measurements is almost certainly due to improper ultrasound technique—specifically incorrect caliper placement—rather than actual testicular shrinkage, especially given your normal sperm count. 1

Why Measurement Error Is the Most Likely Explanation

Technical Factors That Cause Measurement Variability

• Caliper placement errors are the single most common source of inaccuracy in testicular ultrasound measurements, particularly when measuring width (the dimension most prone to error). 1

• Different sonographers using different techniques can produce measurements that vary by 20–30% for the same testicle, which easily explains your 3.1 cm versus 4 cm discrepancy. 1

• The measurements must be taken on three perpendicular dimensions (length, width, height) on axial slices using high-frequency probes (>10 MHz), and many technicians fail to capture the maximum dimension for each measurement. 1

Why Your Normal Sperm Count Rules Out True Atrophy

• Testicular volumes below 12 mL are definitively considered atrophic and are strongly associated with impaired spermatogenesis, reduced total sperm count, and decreased sperm concentration. 2

• If your measurements of 3.1 × 3.4 cm were accurate, your calculated testicular volume using the Lambert formula (Length × Width × Height × 0.71) would be severely atrophic at approximately 7–8 mL. 1, 2

• Men with truly atrophic testes (<12 mL) typically have oligospermia or azoospermia, not normal sperm counts. 3

• Your normal semen analysis strongly suggests your actual testicular volume is in the normal range (15–18 mL), which corresponds to a testicular length of approximately 4 cm. 2

What You Should Do Next

Request a Repeat Ultrasound with Proper Technique

You should request a repeat scrotal ultrasound with explicit instructions to the sonographer to use proper measurement technique. 1

• Specify that measurements should use high-frequency probes (>10 MHz) to maximize resolution and accurate caliper placement. 1

• Request that the sonographer measure three perpendicular dimensions on axial slices and place calipers carefully at the maximum dimension for each measurement. 1

• Ask that the same sonographer perform the measurement if possible to minimize inter-scan variability. 1

• Request that volume be calculated using the Lambert formula (Length × Width × Height × 0.71), not the older ellipsoid formula (0.52 coefficient), which systematically underestimates volume by 20–30%. 2

Compare to the Contralateral Testis

• The report should document both testes measured using identical technique, as a size discrepancy greater than 2 mL or 20% between testes warrants further evaluation to exclude pathology. 1, 2

Why Your Testicles "Feel Smaller"

Subjective Perception vs. Objective Reality

• Subjective perception of testicular size is notoriously unreliable and does not correlate well with actual measured volume. 2

• Factors such as scrotal skin laxity, temperature-related cremasteric contraction, and even anxiety can alter how testicles feel without any change in actual testicular volume. 4

• Your normal sperm count provides objective evidence that your testicles are functioning normally, which would be extremely unlikely if they had truly atrophied. 2, 3

When to Worry About True Testicular Atrophy

Red Flags That Would Suggest Real Pathology

You should pursue further workup only if you have any of the following high-risk features:

• Age under 30–40 years with confirmed testicular volume <12 mL (which carries a ≥34% risk of intratubular germ cell neoplasia if testicular cancer develops). 2

• History of cryptorchidism (undescended testicles), which substantially increases cancer risk and mandates closer surveillance. 2

• Abnormal semen analysis showing sperm concentration <15 million/mL, which would indicate impaired spermatogenesis. 2

• Elevated FSH levels (>7.6 IU/L), which indicate reduced testicular reserve and impaired spermatogenic capacity. 2, 3

• Palpable testicular mass or rapid testicular atrophy on serial examinations. 2

What Testing Is NOT Indicated Right Now

• Hormonal evaluation (FSH, LH, testosterone) is NOT indicated given your normal sperm count, as it would not change management. 2

• Testicular biopsy is NOT indicated in the absence of confirmed small testicular volume (<12 mL), abnormal semen analysis, or high-risk features. 2

• Karyotype testing is NOT indicated unless semen analysis shows severe oligospermia (<5 million/mL) or azoospermia. 2

Bottom Line

Request a repeat ultrasound with proper technique before pursuing any further workup. Your normal sperm count strongly suggests the initial measurements were inaccurate due to technical error, which is extremely common in testicular ultrasound. 1, 2 True biological change in testicular size over 4 weeks is extremely unlikely in adults unless there is acute pathology (such as torsion, trauma, or infection), none of which you describe. 1