Anogenital distance (AGD) is the distance from the midpoint of the anus to the genitalia, the underside of the scrotum or the vagina. It is considered medically significant for a number of reasons, in both humans and animals, including sex determination and as a marker of endocrine disruptor exposure. It is regulated by dihydrotestosterone, which can be disrupted by phthalates common in plastics. Such endocrine disruption may affect the development of the brain.
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 (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. 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 (CDC) 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 (PCOS) had longer AGD, suggesting higher fetal testosterone exposure, than girls born to women without PCOS.
In other animals
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 (DBP) or benzyl butyl phthalate (BBzP).
In 2017, Gobikrushanth et al. studied the relationship between AGD (the distance from the center of the anus to the base of the clitoris) 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. 
The AGD is measured as follows: from the center of the anus to the posterior convergence of the fourchette (where the vestibule begins) 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.
- High Prenatal Exposure to Bisphenol A Reduces Anogenital Distance in Healthy Male Newborns Journal of Clinical Research in Pediatric Endocrinology, 2018
- McEwen, GN; Renner, G (January 2006). "Validity of anogenital distance as a marker of in utero phthalate exposure". Environmental Health Perspectives. 114 (1): A19–20. doi:10.1289/ehp.114-a19b. PMC 1332693. PMID 16393642.
- Welsh, Michelle; et al. (13 March 2008). "Identification in rats of a programming window for reproductive tract masculinization, disruption of which leads to hypospadias and cryptorchidism". Journal of Clinical Investigation. 118 (4): 1479–1490. doi:10.1172/JCI34241. PMC 2267017. PMID 18340380.
- Swan, Shanna H.; et al. (August 2005). "Decrease in Anogenital Distance among Male Infants with Prenatal Phthalate Exposure". Environmental Health Perspectives. 113 (8): 1056–1061. doi:10.1289/ehp.8100. PMC 1280349. PMID 16079079. Lay summary – OurStolenFuture.org: Swan et al., "Decrease in Anogenital Distance among Male Infants with Prenatal Phthalate Exposure".
- Zabarenko, Deborah (4 April 2011). "Key genital measurement linked to male fertility". Reuters. Retrieved 2011-08-24.
- Swan SH, Sathyanarayana S, Barrett ES, Janssen S, Liu F, Nguyen RH, Redmon JB, TIDES Study Team (April 2015). "First trimester phthalate exposure and anogenital distance in newborns". Human Reproduction. 30 (4): 963–972. doi:10.1093/humrep/deu363. PMC 4359397. PMID 25697839.
- Barrett, E. S.; Hoeger, K. M.; Sathyanarayana, S.; Abbott, D. H.; Redmon, J. B.; Nguyen, R. H. N.; Swan, S. H. (January 2018). "Anogenital distance in newborn daughters of women with polycystic ovary syndrome indicates fetal testosterone exposure". Journal of Developmental Origins of Health and Disease. 9 (3): 307–314. doi:10.1017/S2040174417001118. ISSN 2040-1744. PMC 5997496. PMID 29310733.
- Hsieh MH, Breyer BN, Eisenberg ML, Baskin LS (March 2008). "Associations among hypospadias, cryptorchidism, anogenital distance, and endocrine disruption". Current Urology Reports. 9 (2): 137–142. doi:10.1007/s11934-008-0025-0. PMID 18419998.
- Jain VG, Singal AK (September 2013). "Shorter anogenital distance correlates with undescended testis: a detailed genital anthropometric analysis in human newborns". Human Reproduction. 28 (9): 2343–9. doi:10.1093/humrep/det286. PMID 23838161.
- Hood, Ernie (October 2005). "Are EDCs Blurring Issues of Gender?". Environmental Health Perspectives. 113 (10): A670–A677. doi:10.1289/ehp.113-a670. PMC 1281309. PMID 16203228.
- Honma S, Suzuki A, Buchanan DL, Katsu Y, Watanabe H, Iguchi T (2002). "Low dose effect of in utero exposure to bisphenol A and diethylstilbestrol on female mouse reproduction". Reproductive Toxicology. 16 (2): 117–22. doi:10.1016/S0890-6238(02)00006-0. PMID 11955942.
- Gobikrushanth M, Bruinje TC, Colazo MG, Butler ST, Ambrose DJ (2017). "Characterization of anogenital distance and its relationship to fertility in lactating Holstein cows". Journal of Dairy Science. 100 (12): 9815–9823. doi:10.3168/jds.2017-13033. PMID 28941819.
- Salazar-Martinez, Eduardo; Romano-Riquer, Patricia; Yanez-Marquez, Edith; Longnecker, Matthew P.; Hernandez-Avila, Mauricio (2004-09-13). "Anogenital distance in human male and female newborns: a descriptive, cross-sectional study". Environmental Health. 3 (1): 8. doi:10.1186/1476-069X-3-8. ISSN 1476-069X. PMC 521084. PMID 15363098.