All mammals, including humans, start as 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 or XY genetic code, and for the first 5-6 weeks of gestation only the X gene expresses. Then when the Y gene starts expressing (in genetic XY-males), it releases androgens like testosterone, represses some X gene expression (and estrogen development), 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]

Thus, although we can simplify this concept to, “all mammals essentially start as females” when speaking casually and be “sort of correct,” the idea that “all mammals start is females” or specifically “all humans start as females” is not technically correct.

It is more accurate to say, “until the sex-determination process begins, a developing human (technically an embryo) has no anatomic or hormonal sex (just XX or XY genetic code); AKA it is, genetic code aside, a ‘blank slate’.” Then, from there, only the X gene expresses for the first 5-6 weeks of gestation in both XX and XY. During this time, undifferentiated bi-potential structures like the gonadal ridge are developed. Those structures go on to become either “male” or “female” internal and external sex organs starting at about the 5-6 week mark. As, in genetic XY-males, at at the 5-6 week mark, the Y gene expresses, androgens are released, and some X gene expression is repressed (and thus the expression of specific gonadal steroid hormones like estrogen is repressed).”

From there it is a matter of genetic and hormonal expression based on genetic code (a complex process which can produce a range of results including what we call intersex).”

XX vs. XY: Which gene is the male gene and which is the female? In simple terms, the X gene is the gene responsible for “female” features. Meanwhile, in equally simple terms, the Y gene is responsible for repressing “female” features and expressing “male” features.

Bottomline on the question “do all humans start as females”: To rephrase the above before moving on. Since the X gene is what makes a developing human “female,” it is tempting to equate X gene expression in the early stages of embryonic development to the female sex. However, sexual differentiation doesn’t occur until about the 5-6th week of gestation (thus what the X gene is expressing in those first few weeks is really neither male or female). Before the 5-6 week mark, all embryos have undifferentiated structures that will become internal and external sex organs. Gonadal steroid hormones released starting at 5-6 weeks (and a host of complex factors pertaining to this process) dictate how those undifferentiated structures will develop into male or female internal and external sex organs. Thus, a developing human starts as “a blank slate” with undifferentiated structures, and although only the X gene expresses first, it isn’t exactly right to say “all humans start as female” (as sexual differentiation hasn’t occurred yet). Simply put, genetic code aside, all mammals including humans start as a “blank slate” and remain this way for about the first 5-6 weeks of gestation. Learn more about Sex Determination and Differentiation from[6]

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 male/female structures like the 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, and even if they did X and Y aren’t technically “female or male” chromosomes; there are XX males for instance (the development of sex organs is more about hormonal express and reception then the instructions in the genetic code itself).

In other words, not only is making claims about sex complicated in the first few weeks of gestation, it is actually questionable that the binary male/female distinction can be properly made after development either.

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 to illustrate the general point in casual conversation (that is, for the first few weeks of gestation only the X gene expresses).

FACT: Both XX and XY have X genes. Once sexual differentiation occurs, both XX and XY release gonadal steroid hormones which release hormones like the “androgen” testosterone and estrogen. The difference is that, starting at that 5-6 week mark, the Y gene begins to repress some X gene expression and leads to more androgens being produced and recepted (on average).

TIP: When genetic and hormonal expression produces a mix of “male” and “female” features this results in what is known as intersex characteristics.[7]

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 (the title of the video; not a statement we are making). 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.[8]

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.)[9]

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 (hence the “blank slate” theory).

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 (although this is perhaps best expressed as a “blank slate” with only the X gene expressing at first). Thus a slightly more complex version but accurate 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.[10]

  • Why do Men have Nipples? We all start life as a blank slate, thus there are many parts that are common to both sexes, including nipples. Nipples develop before the Y gene is expressed in males, and the Y gene goes on to suppress breast development in “males.” The process begins before any hormones are released, adding to the blank slate theory, but continue before the Y gene expresses (thus there is a female quality). This does not mean that one should attribute nipples to “starting as female” (that is arguably an under-simplification of the truth).[11]
  • Why is it thought that some people are born gay or born transgender? We all start life as being the same sex (essentially a blank slate that can become either sex based on genetic code and other factors). 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. Androgens and Estrogen are present in both sexes, this and a host of other factors (some noted above and some not) offer solid supporting evidence to the argument for intersex (and transgender and even potentially other aspects of LGBT). This is however its own issue with much to discuss.

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. Sex Determination and Differentiation.
  7. This is what Intersex Means
  8. Babies (fetuses) start as females?
  9. Stages of Development of the Fetus
  10. Why do men have nipples?
  11. Development of the Human Breast

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

Leave a Reply to Thomas DeMichele Cancel reply

Your Vote: {{ voteModel || 'no vote' | uppercase }}

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 on


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

JEREMIAH couls on

MY sister still does not belive you can you please shed more light on the subject in a kid understandable way

Thomas DeMichele on

Simple version: All people start as a single cell, a fertilized egg. That egg has all the DNA to make a person. For the first few weeks of embryonic development the structures that become sex organs are undifferentiated. Those undifferentiated structures then begin to form into the (what we consider) male and female internal and external glands around 6 weeks. Prior to this only the gene that creates females (the X gene) expresses.

So there is a short time in every humans embryonic life when they have undifferentiated glands (regardless of XX or XY), then a time when only the X gene expresses (regardless of XX or XY), then a time when sex hormones like testosterone and estrogen (which both sexes have) start to be released depending on genetic code (and some other complicated factors). That then causes the release of hormones (and their reception), which is dictated by the XX and XY (and other factors), which goes on to determine “gender” (up until about the 9th – 12th week).

So even more simply: We all start as a “blank slate,” then only the X gene expresses (in both XX and XY) up to about the 6 week mark, in this time period humans have undifferentiated structures that will become internal and external sex organs, then at around 6 weeks the sexual differentiation process begins (based on genetic code) begins at around 6 weeks. From 6 to 12 weeks the main male and female structures then develop.

I’ll work on getting that down to an even more simple format, but essentially the takeaway is “we start as a blank slate with our genetic code, and only the X gene expresses for the first 6 weeks… thus although only the X gene expresses at first, it isn’t exactly right to say all humans/mammals start as female.”

jamie on
Supports this as a Fact.

I’m sorry but I think it is a fact. Even if humans don’t start out completely female, we do share a blue print that is more female than male. I’m a transgender man, and I cant think of any other reason why I would be like this unless there was a hormonal imbalance in the womb converting my gender from female to male. Furthermore, sex is not permanent, it is only maintained by genes playing a tug of war game. If the genes are altered you will revert sex. Sex isn’t always black and white, unexpected things can happen. Some people have both organs, how can that be possible if humans don’t develop as female by default? The ovaries and testicles are the equivalent of each other, they are practically the same organ they just develop differently. The labia and scrotum are homologous, the penis and clitoris are homologous. Male and female sex organs are made from the same tissue they just develop differently. There is even a male equivalent to the uterus cervix and vagina and it is known as the prostatic utricle. It is a pouch in the prostate. It really frustrates me when people say im making up false information. I have studied this for years and i know what I’m talking about. The fetus may be blank or undifferentiated but having a Y chromosome present does not necessarily mean the fetus will develop into a male. The y chromosome can’t work alone, it needs testosterone. Testosterone is actually what really makes a baby male. I would argue that female is the primary sex and male is second. Sex is evolutionary, and it can be reversed. The default sex in birds is male. Certain fish can change sex in a heartbeat its quote normal for some fish

Thomas DeMichele on

Well first off, anyone who thinks your facts are wrong should do some more research, they aren’t (as far as I know from my own research).

I would only say that we are dipping a little bit into hypothesis theory territory when we try to argue: “female” being the primary gender vs. the blank slate theory where testosterone triggers the undifferentiated structures to become “male.”

With that said, if science came around and shows your theory is correct I wouldn’t be surprised given the way it all works.

Very insightful comment. Thanks for sharing.

Knuf Wons on

I appreciate your stance on this topic, but it doesn’t really account for people who are biologically female and identify as males. I’ve only just begun researching this entire subject, but what I’ve found so far is that there is a lot of research exploring the development of male sexual characteristics and very little exploring the development of female characteristics. There are a number of studies pointing to Testosterone and other factors relating to the development of a male, but I have yet to find a study that points out what hormones need to be present for the development of female sexual traits.

Kerubiel on
Doesn't beleive this myth.

Hello. I’m no expert (correct me if I’m wrong), but I think some facts illustrative to the concept of the initial sexless “blank state” with the potential to develop into either sex were not insisted enough upon in the article. For instance: there are remnants of undeveloped genital structures in BOTH men and women – see mesonephric and paramesonehpric ducts. The mesonephric ducts develop in males and atrophy in females, while the paramesonephric ducts atrophy in males and develop in females. Both male and female bodies carry some traces of the opposite’s sex reproductive system. Doesn’t this prove that the “blank state” we all start from is equipped with the potential structures to become either sex? And that not all basic information for maleness is on the Y chromosome? I’d be grateful for an insight regarding this reasoning, if possible. Thanks.

Thomas DeMichele on

Those are all good notes. With that said, I’ll have have to do a bit of research before I offer my input. In the meantime, perhaps someone else will offer their insight.

Kerubiel on
Doesn't beleive this myth.

Hi again,

Read the last but one paragraph from this article:

It says: “Up until a certain age, boys and girls, as fetuses, are indistinguishable, really, so women retain some remnants of the vas deferens, which is the canal that sperm follows.”

While I’d need some connoisseur’s explanation on this, to me it’s pretty much clear that we really cannot equate the X chromosome with femaleness. Females don’t have an Y chromosome and they still have some remnants of the male reproductive system – and the genetic code for these has to be on the X chromosome, in my understanding. We are not female at all in the beginning, we are bipotential and the X is not a “female” chromosome.

Furthermore, while the clitoris is sexually much more important than male nipples, you could see it this way: the male has nipples because they don’t hinder his survival and the female mold is important for survival of the offspring (breastfeeding), while the female has clitoris because it doesn’t hinder her survival and the male mold is important for reproduction (copulation). Reproduction is 100 % possible without the clitoris, so one could safely claim that the penis is the “primary” organ between the two, in a somewhat similar way the female, and not male breasts and nipples are the “primary” structures. So, I think it’s wrong to state that men have nipples because we all “start life as females”. I think the only accurate statement we can make on the matter is that we all start life as blank states, equipped with the basic structures to develop in either sex.

Thomas DeMichele on

I agree. This page ultimately rates the concept that we start as females as a myth, and conversely concludes that the blank slate theory is the one that makes the most sense.

This is of course the conclusion to research, not the final say on the matter. But I do agree based on research. Nice find on the source.

NOTE: I realize that we had stated incorrectly that nipples implied that we started as females toward the bottom of the page. That isn’t the case. The Y gene suppresses breast growth, and nipples form partly when only the X gene is expressing (although formation begins before any hormones are released), but that doesn’t alone mean “we are born female.” Thanks for catching that. This page has been created over time via research, it is likely that sentence you caught was from the first iteration and not corrected as we learned more.

NOTE: Breast and nipple development is a multi-stage process, for specifics see here:

Joe on
Doesn't beleive this myth.

I have to go with the technical information that states that the fetus starts as a “blank slate”. Of course nipples have to be present in case the fetus becomes a female.

Simon Elliot on

I think calling into question whether the X chromosome is truly female, and whether the hormones can be designated feminizing or masculinising, is a big mistake. Postmodernists and SJWs will have an absolute field day. “Biological foundation for sex differences collapses – social construction hypothesis validated!”

Also, how can the penis be the “primary” sex organ if our species descends from a single-sex ancestor which was female? That is what I read somewhere, that if you go back far enough, there was a point when male had not yet branched off from female.

Julio on

Since when is science not impartial? As far as I see it, how they feel about it is inconsequential. The search for the truth and facts in anything under any circumstance needs to be be done in an unbiased, analytical and objective way.
I admit I do find it mildly curious if not a bit humorous were you read that we descended from a female only and that before males hadn’t “branched off from female” seems to me more like propaganda.

Theresa Miller on
Supports this as a Fact.

There is no “yy”

Thomas DeMichele on

You are right, there is no YY, but there is XYY. The page doesn’t say there is a YY though. /

Awakened on

This is Disinfo

All life starts as Female


Thomas DeMichele on

Actually I explain why this is a misconception above. The idea that all mammals start as female is the claim we are researching, and while there is some logic there, the “blank slate” explanation is much more accurate. As said above, It is more accurate to say, until the sex-determination process begins, a developing human (technically an embryo) has no anatomic or hormonal sex (just XX or XY genetic code); AKA it is, genetic code aside, a “blank slate.”

Mady Maguire on
Supports this as a Fact.

So basically, the only way a male becomes a man is by repressing a female ASAP?

Thomas DeMichele on

Very roughly.

[From what I understand based on my research] What happens is that hormones released by Y gene (the “male” gene) expression repress the hormones released by X gene (“female” gene) expression in XY in the average XY.

However, even in an XY, if the Y gene never expresses (or if something goes “wrong” in the process and say the hormones don’t repress as intended) the person will for all intense and purposes come out with female features.

Just one example of how intersex comes to being and why trying to define sex/gender by what genes a person has is absurd.

Mady Maguire on
Supports this as a Fact.

So to become a man XY must begin repressing XX ASAP?

DPadgett on

I think perhaps many are looking at this in the wrong way, most likely were being confused by focusing far too much on the X and Y chromosome dependency.
As the X, and Y chromosomes may very well be neutral, and act as a storage container for the data being stored.
More importantly I believe we should focus more on fatty acids, and how they determine whether or not particular proteins are to have any effects for characteristic dominance.
While physical dominance may very well be easily decided by biological sex, this does not address the known, and unknown physiology of interrelationships, between the biological instruction and output, along side the influences we miss about the remaining structures of a hierarchy nature.
How exactly does the central nervous system interpolate between the various genetic dominance?
We know that females in general have better immune systems than men.
However we also know that males have a much higher tolerance to pain than that of females, within their nervous system.
We also are aware that a persons DNA changes as one ages.
But let’s also consider afflictions that humans are born with.While we all want to stay fit, and continue to look great when we reach or peak of self perfection, and we know dietary habits play a large roll in whether someone ages well or not.
There however must be an underlying influence within ourselves that makes a male or female at age 30 look 45, while a person age 51 looks still in their 30’s.
It’s my belief the largest influence of development indeed starts to take place after your “Blank Slate” theory.
Perhaps we need to start looking after that point during the development of the endocrine system, which begins around 10 weeks.
We can go back in an attempt to discover what was happening during the Blank Slate period, that influenced the development of the endocrine system.
Considering we know women in general have a better immune system, than males, surely the effect of the immune system on the endocrine system can also help us to detect the association between a biological effect, or end result, via physical attributes.
For me I suspect that biological sex determination is controlled by many variables of systems within the body, in which biological sex determination is dependent far more on a wide variety of systems, rather than merely just being decided by chromosomes.
As I stated prior, it is in my belief chromosomes are merely storage containers.
What I do know is that men are the only of the two sexes that carry both the X and Y chromosomes, that’s a significant consideration.
That’s also why I believe Males came before Females. But what’s important is asking the question of whether a trans gender indeed is psychologically superior in their mental state, however confusing it may be to others, and even themselves.
Superior in the notion that they have somehow been able to tap into a mental association between two opposite biologic sexes.

Thomas DeMichele on

Well, I would under no terms use the term “superior.” It is for one subject, and for two it is a slippery slope full of nasty connotations.

With that said, I generally agree with your line of thinking that says we should look beyond the genes and their containers and look at the development phase, how gene expression affects development, etc.

I’m really not an expert, I simply researched the topic long enough to find a bunch of research that when combined strongly suggested that the blank slate theory was the most accurate.

Dr. khan on
Supports this as a Fact.

Here are four evidences that females were the original genotype, and males evolved later from them:

1) As mentioned here, if no SRY gene is activated on the Y chromosome, all mammal embryos will BY DEFAULT continue on as females.
2) Still today, many amphibians, fish, insects and reptiles are all-female breeding colonies, giving birth to genetic clone females via parthenogenesis and engaging in pseud-coitus. To date, there is no known all-male breeding colony discovered in any species.
3) The female genotype is homologous XX, the male is a deletion mutation of one of the Xs to become a much shorter Y chromosome, later in evolution.
4) The Mitochondrial Eve is 200,000 years old from East Africa, the Genetic Adam is only 60,000 years old, from the same region.

Thomas DeMichele on

Interesting points. Thank you for sharing.

Fuad Khan on

Sure! One more supporting evidence, all juvenile human skeletons are female-like, for both male and female children, and at the time of puberty, under the influence of testosterone surge, the skeleton masculinizes and sexual dimorphism becomes apparent. Once again, the default setting in the early stages of development is female.

Donald Walker on

Not exactly. Both females and males start out as a “thing”, then chromosomes begin to develop characteristics, with the X chromosome beginning before or if the Y chromosome develops.

Thomas DeMichele on

This is a fair way to look at it honestly.

George on
Doesn't beleive this myth.

We start as blank state with potential to be either male or female…people claim femaleness is default but it doesn’t seem to be true.This concept is very old…This all was first identified and elucidated in the 1940s by the French endocrinologist Alfred Jost, who conducted intricate experiments using rabbits and showed that female development is a “default” pathway that needs to be actively overridden for the development of male sex organs….but this concept have been toppled by discovering Such genes like WNT4 which is actively promoting ovarian development and surpressing testicular “programme” or gene called RSPO1…the biggest debunk for this came from certain study from 2017 published in “Science” by Dr.Humphrey Yao. This challenges the age-old concept of the female pathway as “default” and shows that the development of femaleness is also an active process. The authors implicated a protein called COUP-TFII as a key player that is required to actively eliminate the wolffian duct in a developing female embryo in order to give it female characteristics.