The development of biological sex in a fetus is a complex, sequential process that begins at conception and unfolds over many weeks of gestation. This process involves a tightly regulated series of genetic instructions, anatomical changes, and hormonal signals that direct the development of the reproductive system. The timeline is not a single event but a cascade, starting with the establishment of the genetic code. This progression explains why the initial blueprint is set instantly, yet the physical characteristics that define male or female take months to fully manifest.
The Genetic Blueprint at Conception
The biological sex of a developing human is fundamentally determined at fertilization, the instant the sperm meets the egg. This initial step establishes the chromosomal sex, which acts as the ultimate trigger for all subsequent developmental pathways.
If the fertilizing sperm carries an X chromosome, the resulting combination is XX, which typically leads to the development of a female. Conversely, if the sperm carries a Y chromosome, the pairing is XY, which typically results in a male. This genetic composition contains the instructions that will eventually direct the formation of the gonads, either testes or ovaries.
The Early Indifferent Stage
Despite the genetic sex being set from day one, the early embryo remains anatomically indistinct for several weeks. Both genetically male (XY) and female (XX) embryos pass through an “indifferent” stage where their reproductive structures are identical. This period lasts until approximately the sixth week of gestation, during which the embryo possesses a set of dual-purpose internal duct systems.
The embryo develops two precursor duct systems: the Wolffian ducts, which possess the potential to form male internal structures, and the Müllerian ducts, which can develop into female internal structures. The developing gonads, which will become either testes or ovaries, are also in a bipotential state, meaning they have not yet committed to a male or female fate. The body is poised at a developmental crossroads, ready to be directed down either path.
The Hormonal Cascade and SRY Gene Activation
The major shift from the indifferent stage is initiated by the sex-determining region Y, or SRY, gene, located on the Y chromosome. Around the seventh week of development, if the embryo is genetically male (XY), the SRY gene becomes active. This activation triggers a complex molecular cascade that directs the bipotential gonads to differentiate into testes.
Once formed, these embryonic testes begin to produce two hormones that manage the internal differentiation process. The first is Anti-Müllerian Hormone (AMH), which causes the existing Müllerian ducts to regress and disappear, preventing the formation of the uterus and fallopian tubes. Simultaneously, the testes secrete testosterone, which acts to stabilize and develop the Wolffian ducts. These Wolffian ducts mature into the internal male reproductive tract, including the epididymis, vas deferens, and seminal vesicles.
In a genetically female (XX) embryo, the SRY gene is absent. Without the SRY protein, the bipotential gonads follow the default developmental path and differentiate into ovaries. The absence of AMH allows the Müllerian ducts to persist and develop into the uterus, fallopian tubes, and the upper part of the vagina. Similarly, the lack of testosterone causes the Wolffian ducts to naturally regress.
Physical Development of Genitalia
The final stage of sexual differentiation involves the external genitalia, which begins shortly after the internal structures have been determined. The external structures start to differentiate around the ninth week of gestation, driven by the hormonal environment established by the internal gonads. While internal differentiation is complete by the eighth week, the external structures take longer to fully transform.
In the male fetus, the action of testosterone, specifically its derivative, dihydrotestosterone (DHT), stimulates the growth of the genital tubercle into the penis. The surrounding folds fuse to form the scrotum, a process typically complete between weeks 12 and 14. For the female fetus, the lack of DHT causes the genital tubercle to develop into the clitoris, and the adjacent tissue folds remain separate, forming the labia majora and minora.
The anatomical differences become pronounced enough that the fetal sex can sometimes be identified by ultrasound starting around the end of the first trimester, but often more reliably later. The visibility depends on the fetus’s position and the level of development. Full differentiation of the external structures is generally complete by around the 12th week for females and slightly later for males.