Accurately measuring anogenital distance requires a rigid measuring instrument, clearly defined anatomical landmarks, a standardized patient position, and at least two or three consecutive measurements that are then averaged. The measurement itself is straightforward in concept: you are measuring the distance from the center of the anus to a defined genital landmark. But because the landmarks differ between males and females, and because even small technique variations can shift results, getting it right depends on understanding what you are measuring and why each step in the protocol matters.
What AGD Is and Why Precision Matters
Anogenital distance is the span between the anus and a defined point on the genitalia. It has been used for decades in animal toxicology and more recently in human clinical research as a marker of prenatal androgen exposure. AGD is sexually dimorphic: males have a longer measurement than females, with the male-to-female ratio ranging from roughly 1.4:1 to 2.2:1 in humans. That sex difference is already established by 11 to 13 weeks of gestation and reaches its full magnitude by weeks 17 to 20, matching what is seen at birth.1PubMed Central. Anogenital distance as a marker of androgen exposure in humans Because researchers use AGD to detect subtle shifts caused by environmental exposures or hormonal conditions, even a few millimeters of measurement error can cloud results. That makes technique and consistency central to any study or clinical assessment that relies on this number.
The Anatomical Landmarks
AGD is not a single measurement. There are two main variants for each sex, and confusing them is one of the most common sources of inconsistency across studies.
In males, the two standard variants are:
- AGDAS: from the center of the anus to the posterior base of the scrotum, where smooth perineal skin meets the rugated scrotal skin.
- AGDAP: from the center of the anus to the cephalad (upper) insertion of the penis into the body.
In females, the two standard variants are:
- AGDAC: from the center of the anus to the clitoris.
- AGDAF: from the center of the anus to the posterior fourchette (the point where the labia minora meet at the back).
The posterior landmarks (AGDAS in males, AGDAF in females) are generally considered more sensitive to androgen effects, while the anterior landmarks (AGDAP in males, AGDAC in females) capture the full perineal length.2PubMed Central. Anogenital Distance and Perineal Measurements of the Pelvic Organ Prolapse Quantification System Whenever you read an AGD value, the variant matters enormously. AGDAS in adult men is typically much shorter than AGDAP, so comparing a study that reports one variant against a study reporting the other is meaningless unless you know which landmark each used.
Choosing the Right Instrument
The gold standard for AGD measurement in humans is a Vernier caliper, a rigid device with jaws that slide along a calibrated scale. The rigidity is the key advantage. Flexible tape measures introduce slack and curve over soft tissue, widening the margin of error. A study of Turkish newborns noted that flexible tape can increase measurement error and recommended adjustable Vernier calipers specifically because they provide a firm, straight-line measurement.3PubMed Central. Anogenital Distance in Turkish Newborns In practice, most published human AGD research uses digital or dial-face Vernier calipers with precision to at least 0.1 millimeters.
Research in dairy cattle has explored photogrammetry as an alternative, where a photograph of the perineal area is taken alongside a calibrated reference scale, then digitally measured using software. This approach showed moderate correlation with caliper measurements and was repeatable across time points.4PubMed Central. Evaluation of repeatability of anogenital distance measured with either a caliper or photogrammetry Whether photogrammetry will eventually replace calipers in human research remains to be seen, but it is worth noting as a developing option, particularly for field studies where calipers are impractical or where safety around animals is a concern.
Positioning the Subject
How the person being measured is positioned directly affects the measurement. Two positions dominate the literature for adult males: lithotomy (lying on the back with legs in stirrups) and the “frog-legged” position (lying supine with hips flexed and knees apart). A comparison of these two methods in 70 adult men found that they produced similar values for AGDAP but different estimates for AGDAS. Despite that difference, the overall agreement between positions was acceptable, with intraclass correlation coefficients at or above 0.80 for both variants.5PubMed. Comparability and reproducibility of adult male anogenital distance measurements for two different methods The practical takeaway is that either position works for epidemiological studies, but you should not mix positions within the same study and expect clean comparisons.
For newborns and infants, the standard approach is to place the baby supine with the hips flexed at about 90 degrees and the legs relaxed outward. An assistant holds the infant firmly to minimize movement while the examiner takes the measurement. Distraction and gentle engagement with the infant are part of the protocol, because a calm, still baby yields far more consistent readings than one who is squirming.6PubMed Central. Measurement and correlates of ano-genital distance in healthy, newborn infants Trying to measure AGD on a fussy, moving infant is a recipe for unreliable data.
Taking and Averaging Repeated Measurements
A single measurement is never enough. The standard protocol calls for three consecutive measurements at each assessment, with the average of the three used for analysis. This approach smooths out the small positioning variations that inevitably occur even with a cooperative subject and a skilled examiner.7PubMed Central. Anogenital Distance and Penile Length in Infants with Hypospadias or Cryptorchidism: Comparison with Normative Data
How reliable is the result? A large study in male newborns from Mexico reported reliability coefficients for AGD variants between 0.82 and 0.91, which was actually higher than the reliability achieved for stretched penile length or penile width in the same infants.8PubMed Central. Reliability and determinants of anogenital distance and penis dimensions in male newborns from Chiapas, Mexico An earlier, smaller study found lower reliability in both sexes (0.50 for females, 0.64 for males), but noted that between-subject variation was much larger than measurement error, meaning the measurement could still distinguish individuals even when individual readings were less precise.9PubMed Central. Anogenital distance in human male and female newborns: a descriptive, cross-sectional study The difference in reliability between these studies likely reflects examiner experience and training, which brings up an important point: who performs the measurement matters. Having a single, well-trained examiner take all measurements within a study reduces variability compared to rotating among multiple examiners with different levels of practice.
Adjusting for Body Size
A raw AGD number on its own can be misleading, especially in growing infants. A larger baby will tend to have a longer AGD simply because everything is bigger, not because of any hormonal difference. Researchers handle this by creating an “anogenital index,” or AGI, which adjusts the raw distance for body size.
The original and most widely used approach divides AGD by body weight (yielding AGI in mm/kg). This was formalized in a landmark study of prenatal phthalate exposure, where the weight-adjusted index helped separate the effect of chemical exposure from the effect of simply being a bigger baby.10PubMed Central. Decrease in Anogenital Distance among Male Infants with Prenatal Phthalate Exposure In adults, dividing AGD by body mass index has been used instead, since weight alone does not capture adult body proportions as well.11PubMed. Does anogenital distance change with age?
More recent work has questioned whether weight is really the best denominator. A longitudinal study of Mexican children in their first year of life tested several body-size variables and found that height actually produced more precise AGD indices than weight did.12PubMed. Anogenital distance: A longitudinal evaluation of its variants and indices in boys and girls of Sonora, Mexico The debate is far from settled, and different research groups use different denominators, which is yet another reason cross-study comparisons require careful attention to methodology.
In animal research, where AGD measurement is even more routine, guidelines recommend dividing individual AGD values by the cube root of body weight to standardize across animals of different sizes.13PubMed. Rodent anogenital distance recommendations The cube-root transformation accounts for the three-dimensional scaling of body growth in a way that a simple division by weight does not. Human studies have not yet adopted this approach, partly because the statistical models available for human epidemiology handle confounders differently.
Common Pitfalls and How to Avoid Them
Even with good instruments and a clear protocol, several things routinely go wrong in AGD measurement. Understanding these pitfalls is as important as knowing the technique itself.
Landmark misidentification is probably the biggest source of error. In males, the junction between smooth perineal skin and rugated scrotal skin can be ambiguous, particularly in premature infants where scrotal development is incomplete or in adults with significant scrotal laxity. If the examiner is not careful, the caliper tip can drift forward or backward along the perineum, yielding a measurement that is systematically too long or too short. Training with experienced examiners and using standardized photographs of the landmarks help reduce this problem.
Subject movement during measurement introduces random error. This is especially problematic in infants, where a sudden leg kick can shift the caliper mid-reading. The protocol of having a dedicated assistant hold the infant still and keep them calm is not optional if you want reliable data. In adults, simply asking the subject to remain relaxed and not contract the pelvic floor muscles can help, since muscle tension changes the geometry of the perineum.
Mixing measurement variants within a dataset is a surprisingly common mistake in the literature. If some subjects have AGDAS recorded and others have AGDAP, those numbers cannot be pooled. Researchers who switch between variants mid-study, or who fail to specify which variant they measured, produce data that cannot be meaningfully interpreted or compared to other work.
Finally, ambient conditions matter more than you might expect. Temperature affects tissue compliance and muscle tone in the perineal area. Measurements taken in a cold examination room may differ from those taken in a warm one. While no study has rigorously quantified this effect in humans, it is standard advice to ensure a comfortable room temperature and allow the subject to acclimate before measuring.
Why AGD Measurement Accuracy Matters Clinically
The reason all this precision matters is that AGD has become a meaningful clinical and research biomarker across several areas of health.
In pediatrics, shorter AGD in male infants is associated with conditions like hypospadias (where the urethral opening is on the underside of the penis) and cryptorchidism (undescended testes). A meta-analysis found significantly shorter AGD in boys with either condition compared to healthy controls.14PubMed. Associations between hypospadias, cryptorchidism and anogenital distance: Systematic review and meta-analysis A comparison study confirmed that both AGD and penile length were significantly reduced in boys with hypospadias or cryptorchidism compared to healthy controls.7PubMed Central. Anogenital Distance and Penile Length in Infants with Hypospadias or Cryptorchidism: Comparison with Normative Data If AGD is measured poorly, these associations become harder to detect, and the clinical utility of the measurement drops.
In adult men, longer AGD is linked to higher sperm concentration and total motile sperm count at a population level. One study estimated that each additional centimeter of AGD corresponded to about 4.3 million more sperm per milliliter and 6 million more total motile sperm.15PLOS ONE. The Relationship between Anogenital Distance, Fatherhood, and Fertility in Adult Men That said, AGD alone cannot reliably predict an individual man’s fertility. The association holds across groups, not necessarily for any single person.16PubMed Central. Anogenital distance as a measure of human male fertility
In women, AGD differences have been linked to gynecological conditions. A systematic review found that women with polycystic ovary syndrome tended to have longer AGD than controls, while women with endometriosis tended to have shorter AGD.17PubMed Central. A Systematic Review of Anogenital Distance and Gynecological Disorders: Endometriosis and Polycystic Ovary Syndrome These findings are still being explored, but they suggest AGD could eventually serve as a screening clue for certain reproductive conditions in women as well.
AGD in Environmental Health Research
Much of the urgency around standardizing AGD measurement comes from environmental health studies, where researchers track whether prenatal chemical exposures alter genital development. The measurement was originally developed in rodent toxicology, where it became a standard endpoint for detecting the effects of endocrine-disrupting chemicals.18Environmental Science and Pollution Research. Anogenital distance and its application in environmental health research
Translating this to humans has produced mixed results. A 2025 study of prenatal phthalate exposure in infants at 12 months found that higher exposure to certain phthalates was associated with longer AGD in females, but found no significant association in males after correcting for multiple comparisons.19PubMed Central. Prenatal Phthalate Exposure and Anogenital Distance in Infants at 12 Months Meanwhile, a study from Denmark in a population with relatively low phthalate exposure found no significant trend toward shorter AGD in more-exposed boys.20PubMed Central. Prenatal Exposure to Phthalates and Anogenital Distance in Male Infants from a Low-Exposed Danish Cohort (2010-2012) These inconsistencies could reflect real differences in exposure levels, population genetics, or timing of exposure. But they could also reflect measurement variability across studies. When different teams use different landmarks, positions, instruments, and body-size adjustments, the noise in the data can easily swamp the signal.
This is precisely why measurement standardization is such a hot topic. The US Environmental Protection Agency has identified AGD as an endpoint for reproductive toxicity assessment in guidelines, and the push to harmonize human protocols across research centers is ongoing. Until that harmonization is complete, readers of the environmental health literature should pay close attention to the methods section of any AGD study before accepting its conclusions at face value.
Differences Between Rodent and Human Protocols
If you encounter AGD measurement guidelines from animal research, they should not be applied directly to human subjects. Rodent protocols recommend using a microscopic method for late-stage fetuses and early postnatal pups, with the caudal edge of the genital tubercle and the cranial edge of the anus as landmarks.13PubMed. Rodent anogenital distance recommendations These landmarks are not the same as those used in humans, and the scale is completely different. What works under a dissecting microscope on a rat pup does not translate to a caliper measurement on a human infant.
The body-size adjustment also differs. The cube-root-of-body-weight normalization recommended for rodents arose because rodent growth is roughly isometric across early development, making a volumetric correction sensible. Human growth is more complex, with limb proportions, fat distribution, and trunk length all shifting independently. That is why human studies use weight, height, or BMI as denominators rather than a cube-root transformation, and why the question of which denominator is best remains actively debated.
The overlap between animal and human work matters most at the conceptual level. The rodent literature established that AGD is androgen-sensitive and that prenatal exposure to anti-androgenic chemicals shortens it. That concept motivated human studies. But the specific measurement technique, landmarks, instruments, and statistical adjustments all had to be developed fresh for humans, and they are still being refined. Anyone reading across both literatures should treat the two sets of protocols as related but separate.