Testicular Measurement Discrepancy: Which Ultrasound is Correct?
Direct Answer
The measurements showing 3.1cm and 3.4cm length are far more likely to be correct than the 4cm measurement, as a 4cm testicular length would correspond to a volume of approximately 15-18ml, which is inconsistent with your clinical picture of "small testicles" and normal FSH/sperm count. 1
Understanding the Measurement Discrepancy
Technical Factors Causing Overestimation
Caliper placement errors during ultrasound are extremely common and can lead to severely incorrect volume calculations - the 4cm measurement likely included scrotal skin, epididymal tissue, or was measured off-axis rather than at the true maximal testicular dimension 1
The most frequent technical error is measuring width incorrectly by failing to place calipers at the true widest point perpendicular to the long axis, which can artificially inflate all calculated dimensions 1
High-frequency probes (>10 MHz) should be used to maximize resolution and accurate caliper placement - lower frequency probes make precise boundary identification difficult 1
Why 3.1-3.4cm Makes Clinical Sense
Testicular lengths of 3.1-3.4cm correspond to volumes of approximately 10-12ml, which aligns perfectly with your description of "small testicles" - volumes below 12ml are definitively considered small or atrophic 1, 2
Your normal FSH and sperm count are entirely consistent with testicular volumes of 10-12ml, as these volumes typically produce oligospermia rather than azoospermia, with FSH levels remaining in the normal-to-mildly elevated range 1, 3
A true 4cm testicular length (15-18ml volume) would not be described as "small testicles" by any clinical standard - this volume exceeds the 12ml threshold for atrophy and approaches normal adult size 1, 2
Critical Next Steps
Obtain Repeat Ultrasound with Proper Technique
Request a repeat scrotal ultrasound with explicit attention to proper measurement technique, specifically asking the sonographer to measure three perpendicular dimensions (length, width, height) at the maximal dimensions on axial slices 1
The same sonographer should perform serial measurements when possible to minimize inter-scan variability, or measurements should be remeasured on previous scans by the current operator 1
Calculate volume using the Lambert formula (Length × Width × Height × 0.71) rather than the traditional ellipsoid formula (0.52 coefficient), as the Lambert equation provides the most accurate estimates compared to actual testicular volume 1
Compare to Contralateral Testis
Size discrepancy greater than 2ml or 20% between testes warrants further evaluation to exclude pathology, regardless of absolute volume 1
If one testis measured 3.1cm and the other 3.4cm, this represents acceptable normal variation (approximately 0.5-1ml difference), but if one measured 4cm and the other 3.1cm, this would represent a concerning 30-40% discrepancy requiring investigation 1
Clinical Context Matters
Why Your Normal Fertility Parameters Support the Lower Measurements
Men with testicular volumes of 10-12ml (corresponding to 3.1-3.4cm length) typically have oligospermia rather than azoospermia, with FSH levels >7.6 IU/L indicating impaired but not absent spermatogenesis 3
Your normal sperm count confirms that your testicular volumes are in the 10-12ml range - if volumes were truly 15-18ml (4cm length), you would be expected to have completely normal semen parameters without any concern for "small testicles" 1, 4
The Prader orchidometer reading of 15ml is a reasonable estimate for a 4cm testis, though ultrasound would likely measure 16-18ml for the same testis due to systematic measurement differences - this further confirms the 4cm measurement is erroneous 1
Common Pitfalls to Avoid
Don't Average the Measurements
True biological change in testicular size over 4 weeks is extremely unlikely in adults unless there is acute pathology (torsion, trauma, infection) - the discrepancy represents measurement error, not actual size change 1
Averaging 4cm with 3.1-3.4cm would give you a falsely reassuring intermediate value that doesn't reflect reality - one measurement is simply wrong 1
Understand Volume Calculation Errors
Do not use the traditional ellipsoid formula (0.52 coefficient) for clinical decision-making, as it systematically underestimates volume and may lead to inappropriate classification of testicular atrophy 1
Measurements done with calipers on average overestimate testicular length by 5.7mm, thickness by 2.9mm, and width by 1.4mm compared to ultrasound method - this explains how external measurements can be misleading 5
Clinical Significance of 10-12ml Volumes
What This Means for Your Health
Testicular volumes of 10-12ml fall just below or at the 12ml threshold that defines testicular atrophy, with volumes <12ml generally considered small and associated with impaired spermatogenesis risk 1
However, your normal FSH and sperm count indicate you have adequate testicular reserve at present - the combination of borderline-small testes and normal fertility parameters suggests reduced reserve but maintained function 1
Men with volumes at this threshold warrant semen analysis and hormonal evaluation to establish baseline parameters and monitor for decline over time 1
Monitoring Recommendations
Repeat semen analysis every 6-12 months to detect early decline in sperm parameters, as single analyses can be misleading due to natural variability 1
Measure complete hormonal panel (FSH, LH, total testosterone, SHBG) if sperm parameters decline, especially if concentration drops below 20 million/mL 1
Avoid exogenous testosterone or anabolic steroids completely, as these will completely suppress spermatogenesis through negative feedback, causing azoospermia that can take months to years to recover 1