All mammals, including humans, start as female (despite having the genetic XX or XY code to be male or female).

Do All Mammals Start as Female? Do All Humans Start as Female?

Mammals don’t start as females, they start as a blank slate with XX/XY genetic code, and for the first 5-6 weeks of gestation, only the X gene expresses. When the Y gene starts expressing, it releases testosterone, represses some X gene expression, and expresses specific Y genes. This process is called sexual differentiation and it leads to what we call male and female.[1][2][3][4][5]

Although we can simplify this concept to, “all mammals essentially start as females”, this is not technically correct as human sex determination is far more complex than the absence or presence of a Y chromosome.

In Common Language, We Can Say All Mammals Essentially Start as Females, but Technically that Statement isn’t Correct

Using the reasoning above, we can say “all mammals essentially start as females” to keep things simple, but as noted, this not totally correct.

Modern science thinks of the gonads being like a blank slate to start. The gonads then develop into testes and ovaries depending on genetic factors including, but not limited to, the presence of a Y chromosome.

The process is ongoing starting at around 6 weeks, and a complex cascade of genetic products in proper dosages and at precise times is required to for traditional testes or ovaries to form.

The testes or ovaries, or a mix of both in some cases, help determine other the sex organs. Sex organs develop after the body releases hormones which may or may not be absorbed in a particular way if they occur at any given time, and in any doses, and interact in a specific way with a host of genetic factors. There is a lot of room for variation, making simple claims elusive.

Technically humans don’t start female. X and Y aren’t technically “female or male” chromosomes; there are XX males for instance. It’s questionable that the binary male/female distinction can not properly be made after development either. This results in what is known as intersex characteristics.[6] Thus saying all mammals start as female based on a lack of gonadal development and the expression of the X gene only isn’t correct in technical terms, but it is essentially correct in layman’s terms for most of the population.

FACT: All humans start as a fertilized egg containing their genetic code. As the egg develops, gonads start developing as a common primordium (an organ in the earliest stage of development), in the form of gonadal ridges, and only later are differentiated to male or female sex organs. Thus all mammals are born with undeveloped gonads and is thus neither male or female physiologically, despite their genetic code. Then for the first few weeks only the X gene expresses, so in that way mammals are essentially female. Then, a complex cascade of factors occur and the result is sexual differentiation. Learn more about the Development of Gonads and the Stages of Growth: Month by Month.

TIP: See Stanford’s The Genetics of Sex Determination: Rethinking Concepts and Theories for a modern take on sexual differentiation.

We were all female. This is the quick, fun video. Also, see technical video below.

Testosterone and the Y Chromosome

The main culprit here is the traditionally male sex hormone testosterone, which is primarily released from the testes, although its blueprint is primarily contained in the Y gene; the ovaries also release testosterone.

Testosterone both promotes “male” features and restricts “female” features. For example, it restricts the full development of breasts in men to nipple development. It also affects the brain, causing neurological differences in “men” and “women.”

Testosterone also causes the labia to fuse to become the male scrotum. You can see the zipper-like ridge in the middle of the scrotum where the fetal labia joined. The clitoris becomes a penis, the voice deepens, and the prostate gland forms.

Again, in common language, we can say feminine features become masculine due to gene expression related to the Y gene, and only the X gene fires at first, but in technical terms, those statements carry less weight and it’s best to think of the initial pallet as a blank slate.

Sexual Differentiation. This is the technical video.

The Blastocyst, the X Chromosome, and mRNA

Although we say “all fetuses essentially start as female,” it is correct to say “the blastocyst (the fetus before it is a fetus) is developed from the X chromosome, and most early development is driven by maternal mRNA.” RNA consists of bits of genetic code that are being expressed.[7]

Before the blastocyst becomes a blastocyst, it is a single fertilized egg with the baby’s full genetic code. By the time that “a fetus is technically a fetus” (about ten months) the Y gene will have already started expressing and limiting the X gene in males. (Learn more about the Stages of Development of the Fetus.)[8]

So the basic model for humans and mammals comes from the X chromosome, which one can consider “female” in simple terms. It is tempered by the Y gene in males starting at about six weeks, although this isn’t the only determining factor in sex. Before the fertilized egg becomes a blastocyst, it would be hard to decide its sex, even semantically.

As long as we are clear on all the complex factors noted above, we can simplify this to “all humans and mammals start life as females” to transfer knowledge to others without becoming too confusing. Another good, but a slightly more complex version of the factoid is, “the gonads start as a blank slate and develop into either testes and ovaries based on gene expression.”

Reproductive System, part 4 – Pregnancy & Development: Crash Course A&P #43. “Science, where do babies come from?”

Why Does it Matter?

Understanding how mammals develop, and how both genetics (hardwired code) and epigenetics (how that code expresses) affect us, can help us to better understand mammalian, and particularly human nature regarding sex, gender, and sexuality.

The above phenomena help explain the similarities between male and female genitals and orgasms; it explains why men have nipples, and may also explain why sex, sexual identity, and sexual preference are spectrums rather than binary either/or things.[9]

  • Why do Men have Nipples? We all start life as women and nipples develop before the Y gene is expressed in males.
  • Why is it thought that some people are born gay or born transgender? We all start life as being the same sex. As we grow and our genes express, a wide spectrum of influences affects fetal development and the neonate. Many of these factors have to do with the Y chromosome and hormones, which affect the brain. Since a wide variety of influences impact the blastocyst and fetus, it is reasonable to expect a wide spectrum of results in the neonate.

How Sex Genes Are More Complicated Than You Thought. More effects of X and Y chromosomes.


We all have very specific hardwired code from our parents when we start life as a single fertilized egg (zygote), from there it is a matter of gene expression. When the Y gene expresses males become males, until then, only the X gene expresses and we are all essentially females (if you judge female / male by sex organ rather than genetic code).


  1. X chromosome
  2. X chromosome
  3. XY sex-determination system
  4. Sexual differentiation
  5. Stanford’s The Genetics of Sex Determination: Rethinking Concepts and Theories for a modern take on sexual differentiation
  6. This is what Intersex Means
  7. Babies (fetuses) start as females?
  8. Stages of Development of the Fetus
  9. Why do men have nipples?

"All Mammals Start as Female" is tagged with: Cells, DNA, Evolution, Sex. Gender. and Sexuality

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Nicholas Alexander on

The information provided in this article. Is accurate to a certain point. I feel that subject “all mammals start as female” wasn’t exactly answered.

OK. When embryos develop there are two types. There are deuterostomes and protostomes. Protostomes comprise mostly of invertebrate animals, such as insects and worms, while the deuterostomes are vertebrates, like cats, dogs and humans. The first opening to develop in protostomes is the mouth, while the first in the deuterostomes is the anus. Humans, I am sad to say, are definitely deuterostomes. That means, that humans develop the anus first of all openings in the body, and this occurs at about the nine week mark. So what does this mean?

Well, umm. Now that you know this fact you now know two things. Firstly, you know when the anus develops in a fetus. That’s fairly handy to know if you are, like a mad scientist or something. Secondly, and this is the tricky part. It means that at one point in your life, you were nothing but an a-hole, quite literally.

Thomas DeMichele
Thomas DeMichele on


I’ll take another look at the article to see if I can make things clearer regarding sexual differentiation.