Fetal genitalia begin forming around week 4 of pregnancy, when a small bump called the genital tubercle appears. For the first 8 to 9 weeks, male and female embryos look identical in this region. After that, hormonal signals drive the tissue down different paths, and the external genitalia are fully formed by weeks 17 to 18.
The Identical Starting Point
Every embryo begins with the same set of structures regardless of its chromosomes. Around week 4, a small mound of tissue (the genital tubercle) develops near the base of the embryo, along with two sets of folds and swellings on either side. These structures are the raw material for all external genitalia, whether the fetus is XX or XY. Up through about 8 to 9 weeks of gestation, there is no visible difference between male and female embryos. This stretch is called the indifferent stage.
During this same window, two pairs of internal ducts also form side by side in every embryo. One pair will eventually become the uterus and fallopian tubes. The other can become the vas deferens and related male structures. Both sets are present early on, waiting for hormonal signals to determine which pair stays and which disappears.
What Triggers the Split
The divergence starts with the gonads, not the external genitalia. In embryos carrying a Y chromosome, a gene called SRY activates and directs the undifferentiated gonad to become testes. This happens around weeks 6 to 7. The newly formed testes then begin producing two critical signals: testosterone and a hormone that causes the female duct system to break down. Without a Y chromosome and SRY, the gonads become ovaries, and the male duct system degrades instead.
For external genitalia specifically, the key player is a potent form of testosterone. The testes produce testosterone starting around day 60 of development (roughly week 8 to 9), and an enzyme in the surrounding tissue converts it into a more powerful version that reshapes the genital tubercle, folds, and swellings into male anatomy. Without this hormonal signal, the same structures follow a female developmental path by default.
Male External Genitalia: Week by Week
Once testosterone conversion ramps up around weeks 8 to 9, the genital tubercle begins elongating into a penis. The urogenital folds on either side fuse along the underside to form the urethra within the shaft. The labioscrotal swellings, which sit further out, migrate and fuse to form the scrotum. By weeks 17 to 18, the external genitalia are structurally complete, though still small.
The testes themselves have a separate journey. They form inside the abdomen and begin descending in two stages. The first phase, moving them down through the abdomen, starts as early as week 8. The second phase, where they pass through the inguinal canal into the scrotum, begins around week 26. In most fetuses, the testes have settled into the scrotum by week 33. In some boys, particularly those born premature, descent completes after birth.
Female External Genitalia: Week by Week
Female development doesn’t require the same hormonal surge, but it’s not simply a passive process. The clitoris and labia majora become identifiable as early as week 7. By week 8, the labia minora begin forming as ridges of thickened skin flanking the urogenital opening. At week 9, the glans of the clitoris separates as a distinct structure at the tip.
Between weeks 10 and 14, the clitoral hood gradually extends over the glans, starting from the top and folding downward. The clitoral frenulum appears around week 12. After about week 14, the clitoral shaft bends and the labia majora grow to partially cover it. Like male genitalia, the female structures are essentially complete by weeks 17 to 18.
Internal Organs Develop on a Parallel Track
While the external anatomy is taking shape, the internal reproductive organs follow their own schedule. In female embryos, the two internal ducts fuse around week 6 after fertilization to form a combined structure. By roughly week 10, the wall between them dissolves, creating the uterus. The upper portions of these ducts become the fallopian tubes. Problems during this process, particularly around week 5, can lead to structural differences in the uterus.
In male embryos, the hormone released by the testes causes these same ducts to break down. Meanwhile, testosterone preserves and develops the other duct pair into the vas deferens, epididymis, and seminal vesicles. The prostate gland also forms under the influence of that potent testosterone derivative.
Shared Origins, Different Outcomes
Because male and female genitalia develop from the same starter tissues, every structure has a counterpart in the other sex:
- Genital tubercle becomes the penis in males, the clitoris in females
- Urogenital folds fuse to form the penile urethra in males, or remain separate as the labia minora in females
- Labioscrotal swellings fuse into the scrotum in males, or remain separate as the labia majora in females
This shared origin explains why the structures have similar nerve supply and tissue composition despite looking very different at birth.
When Ultrasound Can Identify Sex
Even though differentiation begins around weeks 9 to 10, it takes time before ultrasound can reliably tell the difference. At 11 weeks, accuracy is only about 72% when identification is possible. By 12 weeks, that jumps to 92%, and at 13 weeks it reaches 98.3%. Most providers wait until the anatomy scan around 18 to 20 weeks, when accuracy is essentially 100% and the structures are large enough to see clearly.
Before about 14 weeks, both male and female fetuses have a small protruding structure (from the genital tubercle) that can look similar on imaging. Sonographers assess the angle of this structure relative to the spine to make early predictions, but the margin for error is real at those early dates.
When Development Doesn’t Follow the Typical Path
The process of genital differentiation depends on hormones reaching the right tissues at the right time, and on those tissues being able to respond. When any link in that chain is disrupted, development can diverge from what the chromosomes alone would predict.
One well-studied example is androgen insensitivity syndrome, where an XY fetus produces testosterone normally but the body’s cells can’t respond to it. In the complete form, the external genitalia develop along entirely female lines despite the presence of testes internally. In partial forms, the result can range from mostly female to mostly male external anatomy, depending on how much receptor function remains. These variations highlight that genital development is driven by hormone signaling, not chromosomes directly.
The window of sensitivity for external genitalia is relatively narrow. Animal research has shown that hormone exposure during the period corresponding to roughly weeks 8 through 14 in humans is what shapes external anatomy. Disruptions outside that window affect other systems but leave the external genitalia largely unchanged. This is why certain environmental exposures or hormonal conditions have the greatest impact during the first trimester.