Primary sex characteristics are the reproductive organs and structures that are directly involved in reproduction, present from birth, and distinguish biological males from females. In males, these include the testes, penis, epididymides, seminal vesicles, prostate, and the associated duct systems; in females, they include the ovaries, uterus, fallopian tubes, and vagina.1American Journal of Diseases of Children. Primary and Secondary Sexual Characteristics: Study of Their Development in Males From Birth Through Maturity, With Biometric Study of Penis and Testes Unlike secondary sex characteristics, which emerge at puberty and include features like breast development or facial hair, primary sex characteristics are established before birth and are shaped by a cascade of genetic and hormonal signals that begins surprisingly early in fetal life.
What Counts as a Primary Sex Characteristic
The term “primary” here refers to the organs essential for procreation. In the broadest sense, the gonads themselves, meaning the testes and ovaries, sit at the center of the definition because they both produce the sex cells (sperm and eggs) and secrete the hormones that drive further sexual development. But the definition extends beyond the gonads to include the full internal and external reproductive anatomy. For males, that means the epididymides (where sperm mature), the vas deferens (the ducts that transport sperm), the seminal vesicles and prostate (which produce seminal fluid), and the penis. For females, it means the fallopian tubes (oviducts), uterus, cervix, and vagina.
A common confusion is lumping primary and secondary sex characteristics together. Breast development, widening of hips, deepening of the voice, and growth of body hair are all secondary characteristics. They appear during puberty under the influence of hormones produced by the gonads, but they do not play a direct anatomical role in reproduction. Primary sex characteristics, by contrast, are present in some form at birth, though they remain immature until puberty activates them.
How Primary Sex Characteristics Form Before Birth
For roughly the first six weeks of human development, the embryo’s reproductive anatomy is identical regardless of genetic sex. The embryo has a set of undifferentiated “primordial” gonads that can become either testes or ovaries, plus two sets of internal ducts: the Wolffian ducts (which can become male internal structures) and the Müllerian ducts (which can become female internal structures). A single gene on the Y chromosome called SRY tips this balance.
SRY, identified as the mammalian sex-determining gene over 15 years ago, triggers the primordial gonads to develop into testes.2PubMed Central. SRY and the standoff in sex determination When SRY is expressed in the right cells at the right time, it sets off a chain reaction that causes those cells to become Sertoli cells, which then orchestrate the rest of testis formation.3Encyclopedia of Life Sciences. SRY, Sex Determination and Gonadal Differentiation Without SRY, or without a functioning copy of it, the primordial gonads default toward becoming ovaries.4Springer / PubMed Central. Role of gonadal hormones in development of the sexual phenotypes Calling this a “default” does not mean ovarian development is passive; female reproductive tract formation involves its own coordinated set of genetic signals, including WNT family proteins and region-specific Hox genes that pattern the oviduct, uterus, cervix, and upper vagina.5Frontiers in Cell and Developmental Biology. Mechanistic Drivers of Müllerian Duct Development and Differentiation Into the Oviduct
Once testes form, they produce two hormones that shape the rest of male anatomy. Testosterone from fetal Leydig cells promotes development of the Wolffian ducts into the epididymis, vas deferens, and seminal vesicles. Meanwhile, anti-Müllerian hormone (AMH), produced in large amounts by immature Sertoli cells, causes the Müllerian ducts to regress so that a uterus and fallopian tubes do not form.6PubMed Central. What Does AMH Tell Us in Pediatric Disorders of Sex Development? The external genitalia depend on a particularly potent form of testosterone called dihydrotestosterone (DHT), which drives the differentiation of the genital tubercle and urogenital sinus into the penis, scrotum, urethra, and prostate.7PubMed Central. The effect of 5α-reductase-2 deficiency on human fertility Androgen signaling, particularly DHT-driven regulation of tissue growth, is essential for this external genital development.8Nature Reviews Urology. Regulation of masculinization: androgen signalling for external genitalia development
In the absence of testicular hormones, the Müllerian ducts persist and develop into the fallopian tubes, uterus, cervix, and upper vagina, while the Wolffian ducts degenerate. The external genital tubercle develops into the clitoris and labia rather than a penis and scrotum. The entire process underscores a principle that researchers established decades ago: male phenotypic development requires an active, hormonally functional testis, whereas a female phenotype forms if the gonad differentiates into an ovary or if no gonad is present at all.4Springer / PubMed Central. Role of gonadal hormones in development of the sexual phenotypes
Puberty and the Activation of Primary Sex Characteristics
Although primary sex characteristics exist at birth, they remain functionally immature through childhood. In children, the reproductive organs are considerably smaller and structurally different from adult organs because they have not yet begun their specific reproductive functions.9JAMA Pediatrics. Primary and Secondary Sexual Characteristics: Study of Their Development in Males From Birth Through Maturity, With Biometric Study of Penis and Testes The hormonal axis that controls reproduction, connecting the hypothalamus, pituitary gland, and gonads, is actually active during fetal life and briefly after birth, then goes quiet during childhood. Its reactivation is what triggers puberty.10PubMed Central. Pubertal development and regulation
During puberty, the testes begin producing large amounts of testosterone, which drives growth of the penis, testes, and prostate and initiates sperm production. In females, rising estrogen levels promote growth of the uterus, maturation of the ovaries, and the onset of ovulation and menstruation. These changes transform the primary sex characteristics from dormant anatomical structures into fully functional reproductive organs. The timing varies considerably between individuals, generally beginning between ages 8 and 13 in girls and 9 and 14 in boys, with a wide range of normal variation.
When Development Takes a Different Path
The sequence of genetic signals and hormonal exposures that builds primary sex characteristics has many steps, and variation at any one of them can produce anatomy that does not fit neatly into a typical male or female pattern. These variations, broadly called differences of sex development (DSD) or intersex conditions, are more common than most people realize and illustrate just how many moving parts are involved.
Androgen Insensitivity Syndrome
In complete androgen insensitivity syndrome (CAIS), a person has XY chromosomes and internal testes that produce testosterone, but the body’s cells cannot respond to androgens. The result is that external anatomy develops along female-typical lines: the person has female external genitalia, but no uterus (because AMH from the testes still caused the Müllerian ducts to regress). People with CAIS do not menstruate and cannot conceive.11PubMed Central. Complete Androgen Insensitivity Syndrome in Three Generations of Indian Pedigree CAIS is often not diagnosed until puberty, when menstruation does not begin, or sometimes even later. It is a striking demonstration that having a Y chromosome and functioning testes is not sufficient for male anatomy; the tissues must also be able to respond to the hormones those testes produce.
Congenital Adrenal Hyperplasia
Congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency works in something like the opposite direction. Here, a person with XX chromosomes and ovaries is exposed to excess androgens from the adrenal glands during fetal development. The classic, severe form is the most common cause of atypical genitalia in newborns with a 46,XX karyotype.12The Lancet. Congenital adrenal hyperplasia due to steroid 21-hydroxylase deficiency The adrenal androgen excess regularly causes prenatal virilization of the external genitalia in affected females.13PubMed Central. Perception of women with classic congenital adrenal hyperplasia and their parents on genital surgery and a diagnosis of differences of sex development The internal reproductive organs, ovaries, uterus, and fallopian tubes, typically develop normally because the adrenal androgens do not produce AMH. With hormone management, many people with CAH can menstruate and become pregnant.
5-Alpha Reductase Deficiency
This condition reveals the specific role of DHT in shaping external genitalia. People with 5-alpha reductase type 2 deficiency have XY chromosomes and testes that produce normal levels of testosterone, but they cannot efficiently convert that testosterone into DHT. Because DHT is the androgen primarily responsible for male external genital development during fetal life, affected individuals are often born with ambiguous or female-appearing external genitalia despite having internal testes.14PubMed. 5-α-Reductase type 2 deficiency: is there a genotype-phenotype correlation? A review At puberty, however, the surge in testosterone can drive significant virilization, including growth of the phallus and descent of the testes. This dramatic pubertal change has led some researchers to recommend deferring sex assignment decisions until puberty, when the person can participate in the decision-making process themselves.15PubMed Central. 5-Alpha reductase deficiency; an important cause of 46, XY DSD: Report of three cases within a family
Interestingly, animal research has shown that when the enzyme responsible for converting testosterone to DHT is knocked out, local testosterone levels can rise dramatically to partially compensate. In one mouse study, animals lacking functional 5-alpha reductase type 2 showed DHT levels at only about a fifth of normal in developing external genitalia, but testosterone levels climbed to roughly four times normal, partially rescuing masculinization.16PubMed Central. Possible testosterone redundancy for 5α-dihydrotestosterone in the masculinization of mouse external genitalia This redundancy helps explain the wide range of genital anatomy seen in humans with this condition, from nearly typical female appearance to mild under-masculinization.
Shifting Attitudes Toward Surgical Intervention
For decades, the standard medical approach to newborns with atypical genitalia was to assign a sex quickly and perform early surgery to make the anatomy match. This practice, developed in the 1950s, was built on the assumption that clear-cut physical appearance was necessary for healthy psychological development. Over the past few decades, however, follow-up studies of people who underwent these early surgeries have revealed poor cosmetic and functional outcomes and a high rate of psychological distress.17PubMed Central. Surgery for intersex Advocacy from intersex adults, along with growing evidence that early irreversible surgeries can cause more harm than they prevent, has led many medical bodies to recommend more cautious, patient-centered approaches.
Separately, gender-affirming genital surgeries, including vaginoplasty, phalloplasty, and metoidioplasty, have become increasingly refined as options for transgender individuals seeking to align their anatomy with their gender identity.18PubMed Central. Overview of surgical techniques in gender-affirming genital surgery The medical literature on these procedures has expanded considerably, though long-term outcome data remain limited, and innovations in technique continue to show promising early results for improving depth, function, and complication rates.19Nature Reviews Urology. Principles and outcomes of gender-affirming vaginoplasty These surgeries effectively reconstruct primary sex characteristics using existing tissue, making the biology of genital development directly relevant to surgical planning.
Primary Sex Determination in Other Species
The mammalian system, where a gene on the Y chromosome triggers testis development, is far from the only way organisms determine sex and build reproductive anatomy. Birds use a different system entirely. In chickens and other birds, the key gene is DMRT1, found on the Z chromosome. Males are ZZ and females are ZW, and research has shown that it is the dosage of DMRT1 that acts as the sex-determination switch. When researchers engineered a ZZ (chromosomally male) chicken with only one working copy of DMRT1, the bird developed ovaries instead of testes.20PubMed Central. Primary sex determination in birds depends on DMRT1 dosage, but gonadal sex does not determine adult secondary sex characteristics That finding also revealed something unexpected: in birds, the gonads do not fully control secondary sex characteristics the way they do in mammals, meaning plumage and body shape are partly independent of whether the bird has testes or ovaries.
Even more strikingly, many reptiles and some fish do not rely on sex chromosomes at all. Temperature-dependent sex determination (TSD), first reported in 1966 in an African lizard, has since been documented in some fish, several lizard species, tuataras, numerous turtles, and all crocodilians.21Heredity. Segregating variation for temperature-dependent sex determination in a lizard In these species, the temperature of the nest during a critical window of embryonic development determines whether the gonads become testes or ovaries, and from that point the primary sex characteristics follow accordingly. A careful review of fish species has found that some apparent cases of TSD may actually represent thermal disruption of an underlying genetic mechanism, blurring the boundary between the two systems.22PubMed Central. Temperature-dependent sex determination in fish revisited: prevalence, a single sex ratio response pattern, and possible effects of climate change Climate change adds a practical dimension to this biology: rising temperatures could skew sex ratios in affected species, with potential consequences for population viability.
Environmental Chemicals and Disrupted Development
The fetal hormonal environment that shapes primary sex characteristics is not sealed off from the outside world. Endocrine-disrupting chemicals, substances found in pesticides, plastics, and industrial compounds, can interfere with hormonal signaling during the narrow developmental windows when reproductive organs are forming. Many of these chemicals disturb the endocrine system in organisms exposed during prenatal or early postnatal life, and the effects of such exposure are permanent and irreversible.23PubMed Central. Developmental effects of endocrine-disrupting chemicals in wildlife and humans
One well-studied example is hypospadias, a condition where the urethral opening forms on the underside of the penis rather than at the tip. Research suggests that some environmental chemicals act as anti-androgens, directly interfering with testosterone-related gene expression during the critical period of external genital development.24PubMed. Endocrine disruptors, genital development, and hypospadias Pilot studies have looked at whether bisphenol A (BPA) and related compounds measured in newborns are associated with altered genital measurements. One such study found that higher meconium concentrations of BPA were associated with shorter anogenital distance, and higher levels of both BPA and BPS were associated with shorter stretched penile length, though some results did not reach statistical significance.25PubMed Central. Intrauterine Exposure to Endocrine-disrupting Chemicals and Risk of Hypospadias: A Pilot Study The research is still early-stage, but the pattern is consistent with what is known about how androgen disruption during fetal life can alter the development of primary sex characteristics.
Wildlife populations have offered some of the most dramatic examples. Alligators in contaminated lakes, fish downstream of sewage outflows, and frogs exposed to agricultural chemicals have all shown altered reproductive anatomy or shifted sex ratios. These animal findings serve as a kind of warning system, since the basic hormonal machinery that builds gonads and reproductive tracts is broadly conserved across vertebrates. What disrupts a fish embryo’s gonadal development is, at a biochemical level, not entirely different from what could affect a human one, though the doses and exposure routes obviously differ.