The development of a human fetus involves the precise timing and sequential formation of all body parts, including the reproductive organs. Understanding when and how these structures develop offers insight into human growth within the womb.
The Genetic Foundation of Sex
The very first determinant of a fetus’s sex is established at conception by the chromosomes inherited from the parents. Human cells contain 46 chromosomes, including sex chromosomes: XX for females and XY for males.
The presence or absence of the SRY (Sex-determining Region Y) gene on the Y chromosome acts as a primary switch for gonadal development. If present, SRY initiates the formation of testes from an undifferentiated gonad. Without SRY, ovaries develop.
Early Undifferentiated Development
During the initial weeks of gestation, all embryos share common precursor structures for the reproductive system. Around the fifth week, bipotential gonadal ridges form, which can develop into either testes or ovaries. Two sets of ducts, the mesonephric (Wolffian) and paramesonephric (Müllerian) ducts, are also present.
These structures remain undifferentiated for several weeks. By the sixth week, germ cells migrate to the genital ridges, forming indifferent gonads. Early external genitalia also appear around the third week as mesenchymal cells form cloacal folds, creating the genital tubercle. At this stage, male and female embryos cannot be distinguished by their reproductive anatomy.
Formation of External Structures
The differentiation of external genitalia begins around the seventh to eighth week of gestation, though they remain visibly similar until about the ninth week. Distinct male and female external organs then start to form.
In male fetuses, the genital tubercle elongates to form the penis. The urethral folds fuse along the underside of the developing penis to enclose the urethra, while the labioscrotal swellings merge to form the scrotum. This process is largely complete by around 12 to 14 weeks of gestation.
Conversely, in female fetuses, the genital tubercle develops into the clitoris. The urethral folds form the labia minora, and the labioscrotal swellings develop into the labia majora. By approximately 12 to 13 weeks of gestation, the external female genitalia are also largely formed.
Hormonal Influences
Hormones play a guiding role in directing reproductive system differentiation. Once the SRY gene triggers testes development in male fetuses, these testes begin to produce hormones, primarily testosterone, around weeks seven to eight of gestation. Testosterone is crucial for developing male internal reproductive ducts from the Wolffian system and masculinizing the external genitalia.
Male testes also produce Anti-Müllerian Hormone (AMH). AMH causes the regression of the Müllerian ducts, which would otherwise develop into female internal structures like the uterus and fallopian tubes. In female fetuses, the absence of testosterone and AMH allows the Müllerian ducts to develop into female internal organs, and external genitalia follow the female pathway.
When Sex Can Be Identified
While genetic sex is determined at conception and external genitalia begin forming during the first trimester, visual identification of fetal sex typically occurs later in pregnancy.
The earliest reliable ultrasound detection of sex is generally around 12 weeks, though it can be difficult and less accurate. Accuracy increases significantly between 16 and 22 weeks, with routine anatomy scans around 18 to 22 weeks providing a clear view.
For earlier determination, non-invasive prenatal testing (NIPT) offers an alternative. This blood test analyzes cell-free fetal DNA in the mother’s bloodstream, screening for a Y chromosome (indicating male sex) as early as 10 weeks. NIPT is highly accurate for sex determination.