No. In humans, genetic sex is set at fertilization: an X-carrying sperm makes XX; a Y-carrying sperm makes XY.
Parents hear this claim all the time: that every embryo starts as a girl and only later “switches.” It sounds neat and tidy, but it misses how biology actually works. The chromosomal setup that steers development is fixed the instant one sperm reaches the egg. From that moment, the cells carry either XX or XY. What follows is a months-long choreography where tissues respond to gene signals and hormones to build reproductive organs and other traits.
What “Set At Fertilization” Really Means
Every egg carries one X chromosome. Sperm carry either X or Y. The moment they combine, the embryo’s cells inherit a pair: XX or XY. That pairing doesn’t wobble from week to week; it’s baked into every cell line as they multiply. In most pregnancies, XX development will guide ovarian pathways, and XY will kick off testicular pathways through a switch called SRY on the Y chromosome.
SRY isn’t the only player, but it’s the spark that turns on a program that includes SOX9 and related genes. Those signals push early gonadal tissue toward testes, which then produce testosterone and anti-Müllerian hormone (AMH). Those messengers shape ducts and external structures over the next several weeks. Without SRY and the downstream surge, the default tissue plan heads toward ovaries and the internal ducts that support them.
Early Weeks Share A Common Layout
For a short window, embryos with XX and XY both carry two sets of internal ducts. One set can become the uterus, fallopian tubes, and upper vagina; the other can become the epididymis, vas deferens, and seminal vesicles. Which set stays and which regresses depends on the hormone signals that flow from the developing gonads. That shared starting point fuels the popular myth, but “shared” isn’t the same as “female.” Chromosomes have already been set.
Sex Development Milestones: From Zygote To Fetus
Here’s a compact timeline that shows the key turning points most readers ask about. It keeps the science tight and skimmable.
| Stage | What Happens | Key Signals |
|---|---|---|
| Fertilization (Week 0) | Egg (X) meets sperm (X or Y); chromosomal sex is fixed as XX or XY. | Chromosome pair set; SRY present only with Y. |
| Weeks 4–6 | Gonads are “bipotential” and can form ovaries or testes. | SRY not yet expressed or just ramping up in XY. |
| ~Week 6–7 | In XY, SRY kicks on and tips tissue toward testes. | SRY → SOX9 activation. |
| Weeks 7–9 | Testes (in XY) form Sertoli and Leydig cells. | AMH from Sertoli; testosterone from Leydig. |
| Weeks 9–12 | AMH shrinks the ducts that would form uterus; testosterone supports ducts that form male tract. | AMH, testosterone, DHT pathways. |
| Weeks 12+ | External genitalia finalize along the XX or XY route; internal tracts mature. | Hormone levels and receptor responses guide shape. |
Why The “All Female At First” Line Keeps Spreading
Two ideas get blended. First, both paths begin with shared tissues. Second, without a Y-linked spark and hormone surge, the body follows the ovarian route. People condense those points into a catchy claim. It sounds tidy in conversation, yet it erases the fact that chromosomal sex is locked from day one. Shared tissues do not mean “female by default” in the way the phrase gets used online.
Are Embryos Girls Right From Fertilization? Myth Checked
Let’s tackle common claims you may have heard and compare them to what medical genetics and embryology show.
Claim 1: “Sex Isn’t Set Until The Second Trimester”
Chromosomes decide much earlier. The chromosomal pair arrives at the instant of conception. What people often mean is that external features are not obvious on imaging in the very early weeks. That’s true, but it’s a different point entirely. The physical features need time to respond to gene signals and hormones.
Claim 2: “Everyone Starts With Female Organs”
In the first weeks, embryos carry two internal duct systems. One can give rise to structures linked to ovaries; the other can give rise to structures linked to testes. The hormone pattern decides which set stays. Calling that layout “female” blurs the science. It’s better to say the early plan is shared, then diverges.
Claim 3: “SRY Is The Only Switch”
SRY is the trigger in most XY pathways, yet many other genes and signals make the outcome stick. SOX9 helps fix the testicular route; AMH from Sertoli cells shrinks ducts that would have formed a uterus; testosterone and its derivative DHT shape external structures. In XX, the absence of that cascade steers toward ovaries and the ducts that pair with them.
What Parents Usually Want To Know
When Can A Scan Show External Features?
Early scans can miss or misread small structures. Accuracy improves with time as features grow. That’s separate from chromosomal sex, which never “waits” on a scan. Labs that analyze DNA (such as non-invasive prenatal testing) can assess chromosomal material much earlier than an ultrasound can show shape.
Can Labs Check Chromosomes Before Birth?
DNA-based screens can sample fetal DNA in maternal blood. Diagnostic tests can assess chromosomes from placental or amniotic cells when a health reason calls for it. Speak with a clinician if a provider suggests testing for medical assessment. Screens are not the same as diagnostic procedures, and providers explain that difference during care.
How Do Differences Of Sex Development Fit In?
Some babies are born with chromosomal or hormonal patterns that lead to traits that don’t match the usual XX or XY pathway. Clinicians group these under “differences of sex development” (DSD). Care teams work with families using medical guidelines and the child’s needs. These cases do not change the core point about when chromosomal sex is set. They do remind us that nature has range.
Step-By-Step: From Gene Signal To Visible Traits
1) Chromosomal Set
At the union of egg and sperm, the embryo is XX or XY. That pair sits in every cell lineage that grows out from the first few divisions.
2) The Switch
In XY, the SRY region on the Y chromosome flips the gonad toward testes. Without that push, the tissue heads toward ovaries.
3) Hormone Surge
Testes cells release AMH and testosterone. AMH trims ducts that would have made a uterus and tubes. Testosterone backs the ducts that form the epididymis and related structures. A derivative of testosterone, DHT, shapes external structures.
4) Tissue Response
Cells carry receptors that respond to those hormones. The responses mold ducts and external features in a steady sequence across weeks. Timing matters: if a signal arrives late or receptors don’t respond, structures can form in a different pattern.
Trusted Sources To Read And Share
Two easy-to-read references explain the science behind this topic. The MedlinePlus chromosomes overview explains how the X or Y from the father sets chromosomal sex at conception. For the switch that guides testicular development, see the MedlinePlus Genetics page on SRY. Both are written for the public and align with clinical genetics texts taught in medical training.
Gene And Hormone Cast: Who Does What
It helps to connect names you may hear in articles or from a provider with the role they play in development. Keep this cheat sheet handy while reading research or news stories.
| Name | Role In Development | Route |
|---|---|---|
| SRY | Turns on the testicular program in early XY gonads. | XY |
| SOX9 | Locks in testis formation once SRY lights the fuse. | XY |
| AMH | Causes regression of ducts that would form uterus and tubes. | XY |
| Testosterone → DHT | Supports internal ducts and sculpts external male-typical features. | XY |
| WNT/Beta-Catenin Pathways | Support ovarian development and internal ducts when the Y-linked switch is absent. | XX |
| X-Inactivation | In XX cells, one X largely switches off to balance gene dosage. | XX |
Why Words Matter When Talking About Babies
Every family wants clear, respectful language. “Sex” in this article refers to chromosomal and reproductive anatomy pathways. “Gender” is a different topic about identity and social roles. Parents often bump into both terms online. Separating them keeps conversations precise and lowers confusion during care or screening explanations.
Practical Takeaways For Expectant Parents
Scans And DNA Screens Serve Different Jobs
Ultrasound shows structures. DNA screens read genetic material. A provider uses both tools for health assessment during pregnancy. One can’t replace the other. If a report raises a question, your care team will explain follow-ups and options.
The Myth Can Add Stress—Skip It
Once you know the role of chromosomes and the timeline of tissue changes, the tidy slogan loses its grip. You don’t need it to track a healthy pregnancy. Lean on the simple core: the chromosomal pair is set at fertilization; tissue changes play out across weeks under gene and hormone guidance.
Quick Myth-Vs-Science Recap You Can Share
- Myth: Everyone starts as a girl and XY “changes later.”
- Science: Chromosomal sex is fixed at conception; early tissues are shared, then diverge under gene and hormone cues.
- Myth: Sex “isn’t decided” until the second trimester.
- Science: The decision happens the instant egg and sperm combine; external features take time to grow.
- Myth: One gene handles it all.
- Science: SRY is the spark; SOX9, AMH, testosterone, and receptors carry the plan forward.
When To Ask A Clinician More
If screening or imaging turns up a detail that needs a closer look, teams can order targeted tests, bring in genetics, or follow up with repeat scans. That care aims at health decisions, not slogans. If you’re handed an article that leans on the “all female at first” line, use the references above to check the claims against current genetics and embryology.
Bottom Line Parents Can Trust
Chromosomal sex is fixed at the instant of fertilization. Early tissues look alike for a while, then follow the signal path that fits the chromosomal setup. That’s the whole story behind the catchy myth, told in plain words you can share with friends or family.