Studies show that the human perineum is twice as long in males as in females, but males have more variance. Measuring the anogenital distance in neonatal humans has been suggested as a noninvasive method to determine male feminisation and thereby predict neonatal and adult reproductive disorders. A study by Swan et al. determined that the AGD is linked to fertility in males, and penis size. Males with a short AGD have seven times the chance of being sub-fertile as those with a longer AGD. It is linked to both semen volume and sperm count. A lower than median AGD also increases the likelihood of undescended testes, and lowered sperm counts and testicular tumors in adulthood. Babies with high total exposure to phthalates were ninety times more likely to have a short AGD, despite not every type of the nine phthalates tested being correlated with shorter AGD. Swan et al. report that the levels of phthalates associated with significant AGD reductions are found in approximately one-quarter of Americans tested by the Centers for Disease Control and Prevention for phthalate body burdens. Women who had high levels of phthalates in their urine during pregnancy gave birth to sons who were ten times more likely to have shorter than expected AGDs. A 2018 study by Barrett et al. found that infant girls born to women with polycystic ovary syndrome had longer AGD, suggesting higher fetal testosterone exposure, than girls born to women without PCOS.
There have been extensive studies of AGD effects on animals. In some animals it is routinely measured to determine health. Experiments have demonstrated that in rodent studies this distance is shortened when the mother is exposed to chemicals that are anti-androgenic, such as dibutyl phthalate or benzyl butyl phthalate. Bisphenol A in certain doses increases the AGD of both genders of a study on mice. In 2017, Gobikrushanth et al. studied the relationship between AGD and fertility in Canadian Holstein cows and found that first and second parity cows with long AGD had poorer fertility outcomes compared to those with long AGD.
Measurement
The anogenital index is an index used to measure the AGD. It is computed as the AGD divided by weight . The AGD is measured as follows: from the center of the anus to the posterior convergence of the fourchette in females; and from the center of the anus to the junction of the smooth perineal skin with the rugated skin of the scrotum in males.
