Babies (fetuses) start as females?

[backstab]

Some people I've been talking to about said that babies start out as females the first ~2 weeks(or months, not sure) after conception. This is a 'fact' according to them. I remember like 10 years ago my friend's stepfather told us this 'fact.'

My friend and I agree that the fetus is genetically male or female based on the X or Y chromosome, but they say the time before male genitals start to grow would consider them females. I believe that a fetus less than ~2 weeks(or months, not sure of this figure) would be a blank slate to some extent and then the male or female bits start to grow.

Any ideas on this myth?

[Beejtronic]

I remember someone telling me this once, too. IIRC, it was their explanation for why men have nipples.

I think it depends on your definition of "female." The embryo does not have two X chromosomes, nor does it have female reproductive parts. But, if someone's only definition of female is "does not have a penis," then they would technically correct. Women just love being defined entirely by (a lack of) male traits, BTW.

So, basically, I'd think you were right in thinking that an early male embryo is genderless at best.

[Buckle Up]

They addressed this on House MD, and he said the same thing - "We all start out as girls"

[Liza]

I was told that all fetii are the same until something triggers a woman pregnant with a male to release hormones. Then the male fetus starts developing male sex organs. If the fetus is a female, or in the case of a disorder, the mother doesn't release the hormones.

I think the discussion was about a condition where a person is genetically male, but phenotypically female, all because the hormones weren't released at the proper time.

So, I've heard we all start out the same, then hormones stimulate penis production.

According to wiki it's the fetus who produces the hormones that cause the development, not the mom:
http://en.wikipedia.org/wiki/Fetal_genital_development

[llewtrah]

The Y chromosome is normally enough to make a foetus male*. Abnormal foetuses with XXY or XXXY make-up still end up male.

Many genetics books in the popular science section of bookshops will explain about the female default state.

*Unless you want to get into stuff like androgen insensitivity and sex reversal syndromes where receptors ignore the androgens and the XY foetus continues as female (admittedly I've only been collecting data related to cats, because it explains some of the tortie male cats)

[Cervus]

The sex chromosomes of the fetus (the genotype XX or XY) are present at conception, but it's not until about 5-6 weeks that the expression of the physical sex characteristics (the phenotype, i.e. penis or vulva) begin to manifest themselves. If the fetus has a Y chromosome, at 6 weeks of development a gene on the Y-chromosome called the SRY (sex determining region) is activated, which causes the male characteristics to develop. The potential gonads, which up until now have been non-specific, will develop into testes and a penis based on the trigger from the SRY gene at this time. Once the testes are mature enough, another gene turns on to begin producing testosterone, which causes the internal sex organs (vas deferens, etc.) to develop. Part of the testosterone turns into another hormone which then causes the external sex organs to develop.

If the Y chromosome isn't present, the fetus will develop as a female because there will be no trigger from the SRY to tell it otherwise. I think this is where people get the idea that we all "start as females". Actually, the stages of development for all normal fetuses are the same until about 6 weeks or so, when the sex-chromosomes really become important to development.

The chromosomes are present at conception, but if the SRY gene is not triggered - whether through absence of a Y chrmosome, or a mutation/abnormality - the testes and penis will not develop, or will develop abnormally. Sometimes chromosomal abnormalities result in intersexed individuals with an XY or XXY chromosome but no fully-formed male sex organs.

Hope that helped.

[franjava]

The OP suggestion was also mentioned in Jurassic Park by the scientist that was watching over the Raptor eggs.

[Arrow Tech IV]

I haven't heard anyone mention the androgen wash by name. My understanding is that that is how scientists refer to the bath that a male fetus takes in male hormones. A male fetus that does not receive an androgen wash typically appears to be a female (usually attractive), but without large breasts (not having much testosterone later doesn't significantly increase estrogen) and with no ability to reproduce naturally. My Psychology of Women professor in college joked that most of our thin, attractive supermodels actually fit the phenotype for XY "women". So, ironically, the "standard" for women could actually be as genetically ridiculous as a measurement for a woman's feminity as it appears.

[Muncle]

Originally Posted by Cervus

The sex chromosomes of the fetus (the genotype XX or XY) are present at conception, but it's not until about 5-6 weeks that the expression of the physical sex characteristics (the phenotype, i.e. penis or vulva) begin to manifest themselves. If the fetus has a Y chromosome, at 6 weeks of development a gene on the Y-chromosome called the SRY (sex determining region) is activated, which causes the male characteristics to develop. The potential gonads, which up until now have been non-specific, will develop into testes and a penis based on the trigger from the SRY gene at this time. Once the testes are mature enough, another gene turns on to begin producing testosterone, which causes the internal sex organs (vas deferens, etc.) to develop. Part of the testosterone turns into another hormone which then causes the external sex organs to develop.

If the Y chromosome isn't present, the fetus will develop as a female because there will be no trigger from the SRY to tell it otherwise. I think this is where people get the idea that we all "start as females". Actually, the stages of development for all normal fetuses are the same until about 6 weeks or so, when the sex-chromosomes really become important to development.

The chromosomes are present at conception, but if the SRY gene is not triggered - whether through absence of a Y chrmosome, or a mutation/abnormality - the testes and penis will not develop, or will develop abnormally. Sometimes chromosomal abnormalities result in intersexed individuals with an XY or XXY chromosome but no fully-formed male sex organs.

Hope that helped.

I took a course in developmental biology just about a year ago, and this is correct as far as I can recall, except:
There is another gene, similar to the SRY gene present on the X-chromosome. This gene needs to be turned on (the same way SRY has to) as well. If it isn't turned on, the Mullerian ducts do not develop. This is rare compared to problems with the SRY gene, hence the much more recent discovery. I would be glad to provide more details, unfortunately my notes are in another province.

ETA: I just realized it's pointless to say that the gene needs to be turned on to work. What I mean is that both the SRY gene and the "female" gene take an equal amount of energy to be turned on. The female isn't "passive", and becoming male isn't modifying the base-female. You still go from "it" to male.

[llewtrah]

There are a few rare cases where an aberrant gene on an X chromosome makes an XX foetus phenotypically male.

I recall this came up in documentaries about gender testing in sports. There was a sportswoman who was shocked to find out she is a genetic male. Physically and emotionally she is female.

[Mad Jay]

IMO, in the first 2 weeks, the fetus is as much male/female as the sperm and ovum are. I beleive it's not even called a fetus in the first 2 weeks. It's a blastocyst. It is barely an embryo. Usually, it takes 2 weeks for the blastocyst to implant itself to the uterus.

[Jahungo]

There are two ways to look at this statement that make it true(ish).

One is that a human being is by default female. That is, it is the presence of the Y chromosome that makes one male, not the presence of a second X chromosome making one female. So someone whose genotype is XO will be effectively female. And the genes in the Y chromosome do not kick on immediately, but only at some point in development. Therefore, there is no big difference between a male and female embryo at this point, and because female is the "default," one could say that te embryo is female. Although that's a stretch in itself.

Also, in fact most early development is driven by maternal mRNA, which is obviously "female" RNA as it comes from te female parent.

[Jahungo]

Originally Posted by Mad Jay

IMO, in the first 2 weeks, the fetus is as much male/female as the sperm and ovum are. I beleive it's not even called a fetus in the first 2 weeks. It's a blastocyst. It is barely an embryo. Usually, it takes 2 weeks for the blastocyst to implant itself to the uterus.

A blastocyst is a specific term for a ball-like structure that occurs very early in development. I believe that at two weeks an embryo has generally moved past this stage. It is an embryo, as it is basically once conception begins. It is a very young embryo, but nonetheless an embryo.

It is not yet a fetus, which is a term that IIRC begins to apply somewhere around 10 weeks.