Distance from midpoint of the anus to the genitalia
Anogenital distance (AGD) is the distance from the midpoint of the
anus to the
genitalia, the underside of the
vagina, the
clitoris or the
scrotum. It is considered medically significant for a number of reasons, in both humans and other animals, including
sex determination and as a marker of
endocrine disruptor exposure.[1] It is regulated by
dihydrotestosterone, which can be disrupted by
phthalates common in plastics.
The anogenital index (AGI) is an index used to compare the AGD relative to a model based on body weight.[2] It is computed as the AGD divided by weight [AGI = AGD/weight (mm/kg)].[3]
Measurement
The AGD is usually measured as follows: from the center of the
anus to the
posterior convergence of the
fourchette (where the
vestibule begins) in females (AGD-AF); and from the center of the
anus to the junction of the smooth
perineal skin with the rugate skin of the
scrotum in males (AGD-AS).[4] An alternative measurement, AGD-AC, defined only in females, is measured from the anus to the clitoris.[5]
In humans
Early studies showed that the human
perineum was half as long in females as in males, but it has since been found to be three quarters the male distance in females,[6] although males have more variance. Measuring the anogenital distance in neonatal humans has been suggested as a noninvasive method to determine male
feminisation and female
virilization and thereby predict neonatal and adult
reproductive disorders.[7]
A study by Swan et al. determined that the AGD is linked to fertility in males, and penis size.[3] Males with a short AGD (lower than the median around 52 mm (2 in)) 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.[8] 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.[3]
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 (CDC) for phthalate body burdens.[3]
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.[9]
A 2018 study by Barrett et al. found that infant girls born to women with
polycystic ovary syndrome (PCOS) had longer AGD, suggesting higher fetal
testosterone exposure, than girls born to women without PCOS.[10]
Bisphenol A in certain doses increases the AGD of both genders of a study on mice.[13]
In 2017, Gobikrushanth et al. studied the relationship between AGD (the distance from the center of the anus to the base of the vagina and the clitoris) and fertility in Canadian Holstein cows. They found that in first and second parity cows an increase in AGD has an inverse correlation with pregnancy success after first artificial insemination. This correlation was not found in third+ parity cows.[14]