Fetal Sex Development By Week | From Chromosomes To Anatomy

Fetal sex development by week follows a precise biological sequence. It starts the moment sperm meets egg, yet visible differences take months to appear. Early embryos share the same basic structures. Only later do chromosomes, genes, and hormones guide those tissues toward male or female anatomy.

If you’re tracking pregnancy milestones or reviewing medical information, understanding this timeline helps you separate myths from biology. Sex isn’t determined by cravings, heart rate, or belly shape. It’s directed by genetic signals and hormonal shifts that unfold in stages.

How Biological Sex Is Determined At Conception

Sex is established at fertilization. The egg always carries an X chromosome. Sperm carry either X or Y. When an X-bearing sperm fertilizes the egg, the embryo is XX. When a Y-bearing sperm does, the embryo is XY.

The presence of a Y chromosome introduces a gene called SRY (sex-determining region Y). This gene triggers the formation of testes. Without SRY, the embryo develops ovaries. The MedlinePlus Genetics entry on the SRY gene explains how this single gene initiates a cascade of changes that shape sexual development.

Still, chromosomes alone don’t create visible differences. During the first weeks, all embryos look the same externally. Internal structures are also undifferentiated. The roadmap exists, but it hasn’t yet been executed.

Fetal Sex Development By Week: Early Embryonic Stage

The first trimester carries most of the structural groundwork. Organs form. Hormones begin to circulate. Subtle shifts set the stage for either male or female anatomy.

Weeks 1–4: Chromosomes And Early Cell Division

During the first month, rapid cell division forms a blastocyst, then an implanted embryo. Sex is genetically set, but there are no reproductive organs yet. All embryos contain two paired duct systems: the Müllerian ducts and the Wolffian ducts. Each embryo has the potential to develop either pathway.

Weeks 5–6: The Bipotential Gonad

By week five, a structure called the gonadal ridge forms. It’s “bipotential,” meaning it can become either testes or ovaries. At this stage, embryos are indistinguishable in terms of sex.

External genital structures are also neutral. A small genital tubercle, urethral folds, and labioscrotal swellings appear in all embryos. These tissues will later differentiate.

Week 7: The SRY Signal In XY Embryos

In embryos with a Y chromosome, the SRY gene activates around week seven. This prompts the gonads to develop into testes. Once formed, fetal testes begin producing testosterone and anti-Müllerian hormone (AMH).

AMH causes the Müllerian ducts to regress. Testosterone preserves and develops the Wolffian ducts into male internal structures such as the epididymis and vas deferens. The NCBI overview of sexual differentiation outlines how these hormones guide internal development.

In XX embryos, there is no SRY activation. The gonads develop into ovaries. Without AMH, Müllerian ducts persist and later form the uterus, fallopian tubes, and upper vagina.

Weeks 8–9: Hormones Shape Internal Anatomy

By week eight, hormone production intensifies in XY fetuses. Testosterone influences the transformation of Wolffian ducts. In the absence of testosterone, those ducts fade in XX fetuses.

Internally, differentiation is underway. Externally, differences remain subtle.

External Genital Development Across The First Trimester

While internal organs are forming, external genitalia follow a separate timeline. Both XX and XY embryos start with identical structures. Hormones determine how these tissues change.

In XY fetuses, testosterone converts to dihydrotestosterone (DHT), which drives male external development. The genital tubercle elongates to form the penis. Labioscrotal swellings fuse to become the scrotum.

In XX fetuses, without high androgen levels, the genital tubercle becomes the clitoris. Labioscrotal swellings form the labia. The urethral folds remain separate.

According to the American College of Obstetricians and Gynecologists fetal development timeline, visible external differences usually become clearer near the end of the first trimester, though imaging accuracy still varies.

Week-By-Week Milestones In Fetal Sex Development

Below is a consolidated timeline of fetal sex development by week, summarizing key genetic, hormonal, and anatomical events.

Week	Internal Development	External Development
1–2	Fertilization establishes XX or XY chromosomes	No genital structures present
3–4	Early embryonic layers form	Undifferentiated tissue
5	Bipotential gonadal ridge appears	Genital tubercle forms
6	Müllerian and Wolffian ducts present in all embryos	External genitalia identical
7	SRY activates in XY; testes begin forming	No visible difference yet
8–9	Testosterone and AMH guide internal male structures	Genital tubercle begins length change in XY
10–11	Ovaries developing in XX; Müllerian ducts persist	External differences emerging
12	Internal reproductive organs largely defined	Penis or clitoris distinguishable on ultrasound in some cases

Second Trimester: Refinement And Visibility

By weeks 13 to 20, fetal sex development becomes more visible on ultrasound. Internal structures are already established. External genitalia continue maturing.

Weeks 13–16: Growth And Positioning

In male fetuses, the penis grows and the scrotum becomes more defined. Testes remain inside the abdomen at this stage.

In female fetuses, the uterus and ovaries continue development. Ovarian follicles begin forming early egg cells. This process starts before birth and continues until menopause.

Weeks 18–20: Ultrasound Identification

Mid-pregnancy anatomy scans often identify fetal sex with good accuracy. The CDC’s information on congenital genital differences notes that structural variations can affect how genitalia appear, which is why ultrasound interpretation isn’t perfect.

Accuracy improves with gestational age, fetal position, and technician skill. Earlier scans carry higher uncertainty.

Third Trimester: Final Stages Of Reproductive Development

The final trimester focuses on growth and final positioning rather than new differentiation.

In male fetuses, testes typically descend into the scrotum between weeks 28 and 35. This descent is hormone-driven and can continue shortly after birth.

In female fetuses, reproductive organs are structurally complete. Hormonal exposure from the mother may cause temporary genital swelling at birth, which resolves naturally.

Hormones And Their Role In Sexual Differentiation

Hormones direct much of fetal sex development by week. Testosterone, DHT, and AMH are central in male development. In their absence, the body follows the ovarian pathway.

Here’s a simplified comparison of hormone influence:

Hormone	Produced In	Primary Effect
SRY Gene Product	XY Embryo	Triggers testes formation
Anti-Müllerian Hormone (AMH)	Fetal Testes	Regresses Müllerian ducts
Testosterone	Fetal Testes	Develops Wolffian ducts
Dihydrotestosterone (DHT)	Converted From Testosterone	Forms male external genitalia
Estrogen	Placenta And Maternal Source	Supports overall fetal growth

When Variations Occur In Fetal Sex Development

Sex development does not always follow the typical XX or XY pattern. Differences in sex development (DSDs) can occur due to chromosomal variations, gene mutations, or hormone pathway differences.

Examples include androgen insensitivity syndrome, congenital adrenal hyperplasia, and Turner syndrome. These conditions affect how chromosomes and hormones interact with developing tissues.

The NIH Genetic and Rare Diseases Information Center overview of androgen insensitivity syndrome explains how an XY fetus can develop female external characteristics if cells cannot respond to androgens.

Such variations are medical conditions, not lifestyle outcomes. They reflect complex biological pathways.

Common Myths About Fetal Sex Development

Many beliefs circulate about predicting sex early in pregnancy. Most have no scientific basis.

• Heart rate theory: Fetal heart rate does not reliably indicate sex.
• Craving patterns: Food preferences do not correlate with chromosomal makeup.
• Belly shape: Abdominal appearance depends on body type and fetal position.

Only genetic testing, such as noninvasive prenatal testing (NIPT), or anatomical imaging later in pregnancy can provide reliable information.

Why The Week-By-Week Timeline Matters

Understanding fetal sex development by week clarifies when structural changes actually happen. It also explains why early ultrasound predictions can be uncertain.

Most visible differences appear after week 12. Before that, embryos share the same external template. Internally, hormonal shifts begin earlier but remain unseen.

This timeline also guides clinical evaluation. If a newborn presents with ambiguous genitalia, doctors review genetic tests, hormone levels, and developmental timing to understand what occurred during gestation.

Clear Takeaway On Fetal Sex Development By Week

Sex is set at fertilization through chromosomes, shaped in early weeks by gene activation, and refined through hormone-driven tissue changes. The process unfolds in stages, with most visible differences appearing near the end of the first trimester and becoming clearer in the second.

Each week adds a layer: genes signal, gonads form, hormones circulate, ducts regress or persist, and external anatomy develops. The sequence is orderly, measurable, and well documented in medical literature.