Houston, TX, USA. The distance between a man's scrotum and his anus, the anogenital distance (AGD), may indicate his ability to reproduce, report researchers.

According to Dr. Michael Eisenberg, "There are two main implications of this study — first, this could represent a non-invasive way to test testicular function and reproductive potential in adult men and second, it suggests that gestational exposures and development may impact adult testicular function."


AGD, a sexually dimorphic measure of genital development, is the distance from the anus to the base of the penis or vagina. As shown in animal studies, AGD is regulated by dihydrotestosterone, and can be used as a marker for endocrine disruption by phthalates common in plastics. It is known that such endocrine disruption also may affect brain development.

In the specific case of AGD, the shortened distance measurements have been observed in infant males with a risk of testicular dysgenesis syndrome, genital anomalies that include both hypospadia and cryptorchidism. As a consequence, measuring the distance in neonatal (or newborn) humans has been suggested as a noninvasive method to determine male feminisation and thereby predict neonatal and adult reproductive disorders.

Michael Eisenberg is a male reproductive medicine and surgery fellow in the Scott Department of Urology at the Baylor College of Medicine (BCM), and lead author on the study. Referring to previous studies with mice, Eisenberg says "We have observed in animal studies that anogenital distance (the distance between the scrotum and anus) is an important measure for genital development and may be shorter in males with abnormal testicular development and function."

The research team sought to determine if AGD varied in fertile, compared to infertile, adult men. "We initiated a clinical study to determine if the distance varied between infertile men and fertile adult men. If so, this could help us develop a novel method to evaluate patients with impaired reproductive potential."

The research results appear in the journal PLoS ONE. Eisenberg and his team measured the anogenital distance and penile length of 117 infertile men (mean age: 35.3±17.4) and 56 fertile men (mean age: 44.8±9.7) from an andrology clinic.

• The infertile men possessed significantly shorter mean AGD and penile length (PL) compared to the fertile controls (AGD: 31.8 vs 44.6 mm, PL: 107.1 vs 119.5 mm, p<0.01).
  
  
• There was a difference in the AGD even after accounting for ethnic and anthropomorphic (or behavioral) differences.
  
  
• Moreover, AGD was significantly correlated with sperm density and total motile sperm count. For each 1 cm increase in a man's AGD, the sperm density increases by 4.3 million sperm per mL (95% CI 0.53, 8.09, p = 0.03) and the total motile sperm count increases by 6.0 million sperm (95% CI 1.34, 10.58, p = 0.01).
  
  
• On adjusted analyses, no correlation was seen between penile length and semen parameters, consistent with a large body of research that shows fertility unrelated to penile measurements.

Thus, the researchers found that infertile men possessed a significantly shorter anogenital distance and penile length when compared with the fertile men. Since a longer anogenital distance is associated with fatherhood, it may predict normal male reproductive potential.

The authors also note the need for further studies to compare anogenital distance measurement techniques and assess their accuracy and reproducibility.