Prenatal Androgens Deep Dive

Starting Point: You Were Sexually Ambiguous

For the first 7–8 weeks of gestation, all human embryos look anatomically identical regardless of sex chromosomes. Everyone has the same structures:

• A genital tubercle that will become either the penis or clitoris
• Urogenital folds that will become either the penile shaft skin or labia minora
• Labioscrotal swellings that will become either the scrotum or labia majora
• Internal Müllerian and Wolffian ducts, one set of which will persist and develop, the other of which will regress

At week 7 of gestation, looking at ultrasound, you cannot reliably tell whether a fetus is male or female anatomically. The differentiation starts around week 8, when — in embryos carrying a functioning SRY gene (typically on a Y chromosome) — the gonads begin developing into testes instead of ovaries.

The Testosterone Kickoff

Once testes form, they immediately begin producing testosterone. The fetal Leydig cells (testosterone-producing cells) are pumping out hormone starting around week 8. But here's the critical twist: testosterone alone doesn't build the penis. It has to first be converted into a more potent androgen called dihydrotestosterone (DHT).

DHT: The Architect

DHT is made from testosterone by an enzyme called 5-alpha reductase type 2, encoded by the SRD5A2 gene. This enzyme is expressed specifically in genital tissues during fetal development. Testosterone circulates through the entire body; DHT is made locally, right where it's needed.

DHT is roughly 2–3 times more potent than testosterone at binding and activating the androgen receptor (AR). And it's DHT, not testosterone, that drives the fundamental construction of:

• The elongation and growth of the genital tubercle into the penis
• Fusion of the urogenital folds to form the penile urethra
• Fusion of the labioscrotal swellings to form the scrotum
• Descent of the testes from the abdomen into the scrotum (later in gestation)

This is why we know DHT matters so much: when this pathway is broken by genetic mutation, the consequences are severe and specific.

The Natural Experiment: 5-Alpha Reductase Deficiency

Some individuals are born with mutations in the SRD5A2 gene that make their 5-alpha reductase enzyme non-functional. The condition is called 5-alpha reductase type 2 deficiency, and it's been well-characterized clinically.

People with this condition have:

• Normal XY chromosomes
• Normal testes, producing normal amounts of testosterone
• But no functioning enzyme to convert testosterone to DHT in genital tissues

The result: at birth, their external genitalia are severely undermasculinized — the penis fails to form normally, the urethra doesn't fuse properly, and the labioscrotal swellings remain more labia-like than scrotum-like. Many children with this condition are initially assigned female at birth and only identified as having XY chromosomes later.

At puberty, the massive testosterone surge can partially masculinize these individuals even without the DHT pathway — they often develop a more male-typical body shape, deepened voice, and muscle development. But the external genital development that didn't happen in the 8–14 week fetal window can't be meaningfully recovered later. The window closed.

The Androgen Receptor: The Other Half

Making DHT doesn't matter if the tissue can't respond to it. Androgen response requires a functioning androgen receptor (AR), coded by the AR gene on the X chromosome. When DHT binds the androgen receptor, the receptor moves into the cell nucleus and activates specific genes that drive growth and differentiation.

Several things can affect AR function:

• Complete androgen insensitivity syndrome (CAIS): AR doesn't work at all. XY individuals develop externally as female because no tissue can respond to androgens, despite normal testosterone and DHT.
• Partial androgen insensitivity: AR partially functional. Results in a spectrum of undermasculinization depending on severity.
• CAG repeat length variants: Normal AR genes have a variable number of CAG trinucleotide repeats. Shorter repeats are associated with slightly more sensitive receptor function; longer repeats with slightly less. This is a normal variation, not a disorder, and may contribute to individual variation in androgen responsiveness.

So even with identical testosterone and DHT levels, two fetuses can respond differently based on their specific AR gene variant. And — important detail — since the AR gene is on the X chromosome, men inherit it from their mothers, not their fathers. (This is one of the reasons the "you get your size from Dad" folk wisdom falls apart.)

The Timeline, Week By Week

Anogenital Distance: The Measurable Proxy

One fascinating research thread has focused on anogenital distance (AGD) — the distance between the anus and the genitals, measured in infants. In male newborns, this distance is typically longer than in females, reflecting the effect of prenatal androgens on perineal development. Work by Dr. Shanna Swan and other endocrine disruption researchers has shown that AGD in male infants:

• Correlates with measures of prenatal androgen exposure
• Tracks into adulthood with measures like sperm quality and testicular size
• Can be affected by certain prenatal environmental exposures — particularly phthalate plasticizers, which interfere with androgen signaling during gestation

This body of research is significant because it shows that prenatal androgen exposure isn't just determined by your genes — it's also affected by the uterine environment, which in turn is affected by maternal exposures to certain chemicals in the weeks when fetal development is happening. It's a reminder that "genetic" and "determined at birth" aren't the same thing: some of what shapes your development is environmental, just environmental at a stage you couldn't control.

What This Means for You Now

Three practical takeaways from this embryology:

1. Your adult size was substantially determined before you were born. Not entirely — puberty still matters, as do body composition effects that change visible length. But the core trajectory was set by fetal androgen exposure during a window that closed 20-plus years before you ever thought about your size.
2. You didn't do anything wrong. Whatever size you ended up, it was the result of a prenatal biochemical cascade you had zero control over — enzyme efficiency, receptor sensitivity, hormone levels, uterine environment, maybe chemical exposures. None of this reflects anything you did or didn't do.
3. Adult interventions mostly don't work the way the fetal window did. This is why the supplement industry, the extender industry, the pumping industry, and so on don't produce meaningful anatomical length gains in adults. The machinery that made your penis isn't reactivatable. What's done is done, and what's fine is fine.

Bottom Line

The fundamental architecture of your penis was laid down by DHT, produced from testosterone by 5-alpha reductase type 2, acting on androgen receptors in genital tissues, during gestational weeks 8–14. Genetic factors (AR gene variants, SRD5A2 function), maternal factors, and prenatal environmental exposures all modulate how that process runs. Once that window closed, continued growth was shaped by late gestation, minipuberty after birth, and puberty — but the trajectory was set. This is why your adult size isn't something you achieved or failed to achieve. It's something that happened to you, before anyone knew you existed, in a specific biochemical window. You're not a result; you're an outcome.