Do all babies start off female?

By Matt Kaiser

Some of my partner’s friends had been debating whether or not all babies “start off as females”. This debate arose, I guessed while desperately trying to recall my high school biology classes, because of the fact that all embryos follow the same developmental path, regardless of the genetic make-up. This is until certain genes on the Y chromosome (if present) are activated at around eight weeks and male-associated hormones, chiefly testosterone, are produced that act on some cells to trigger the formation of male-specific organs. Without these hormones kicking in, which is the case in XX embryos where no Y chromosome in present, the cells in the developing embryo go on to form female-specific organs. This means that fertilised embryos under normal circumstances will develop female-specific sex organs unless a hormone cue is activated that signals otherwise.

So all embryos are female, then?

Hmm, perhaps not. One can still make a genetic distinction between males and females at the very point of fertilisation, i.e. whether the fertilised zygote contains two X chromosomes or one X and one Y, and this remains static throughout an individual’s development (and, indeed, life). And we know that, due in large part to the hormonal changes I’ve already mentioned, XX zygotes will give rise to females and XY will lead to males.

So embryos are either male of female, then?

Well, it’s not quite as simple as that. There are some clinical oddities that throws some confusion into the mix. Some males, for instance, have two X chromosomes but develop as males instead of females because of the presence of a third, Y chromosome that contains the genes to provide the male hormone cues (“Klinefelter’s syndrome“).

So could we define maleness as the presence of at least one Y chromosome (some males are XYY too)?

Not really, because that definition comes unstuck when we consider individuals who are XX but develop as males (at least outwardly), due to the gene for the male hormone cue being copied to one of the X chromosomes (“XX male syndrome“), or individuals who are XY but develop as females, due to a defective Y chromosome (“Turner syndrome“) or mutated Y genes (“Swyer syndrome“).

This leads to complications when trying to enforce a purely genetic definition of gender. The International Olympics Committee for years attempted to enforce this view to adjudicate on cases of gender uncertainty, believing that this represented a more definitive and less intrusive test than physical examination. In this Y-centric definition, without ever meeting an individual and having only a few of their cells, the presence of the Y chromosome or any of its genes (such as the male sex determining gene called ‘Sex-determining Region Y’ (SRY)) would lead to the conclusion that those cells came from a man.

However, for the reasons I outlined above about all the genetic uncertainties, pressure from a number of medical associations in the USA thankfully led to this sort of test being dropped by 2000. A ‘one-or-the-other’ test of this sort simply does not fully account for the complexities of gender and can lead to discrimination and unfair impediment, as revealed by Chris Cooper in a recent Observer article.

The lead opponent of such gender screening, Georg Facius, even proposed a ‘third gender’ for those that could be considered both male and female. One instance where this could be applicable is in cases of “androgen insensitivity syndrome“, in which XY individuals show abnormal responses to the masculinising hormone androgen. Because the effects can vary, some individuals are anatomically male but have reduced fertility (mild), some possess ambiguous genitalia (partial), while some are almost indistinguishable from XX females (complete).

And this is all before we get into the complex world of gender identity, such as when an individual of one gender is uncomfortable being associated with that gender, which may be environment-driven (or may not be, or may be a little bit). Nor have I touched on psychological and behavioural differences, which lie on a continuous and overlapping spectrum between males and females and are often socially defined (and therefore subject to variation and change).

So none of this means that an embryo is female before the male signals kick in. It is perhaps more accurate to say that an embryo is gender-neutral, i.e. neither male nor female, until towards the end of the embryonic period, at which point anatomical differences start to become apparent in the foetus. But even then, as highlighted above, someone may share characteristics of both sexes and remain ‘double gender’.

Which is all a long-winded of way of saying that a binary male-female distinction is a little fuzzy.

This post is a re-working of a version originally posted at The Skeptical Dad.

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0 Responses to Do all babies start off female?

  1. I never thought that the issue was about whether all babies start off female, but that all embryos begin forming the same way to begin with and that pre-hormone default formation includes something which is only required in females – hence the presence of nipples on both males and females. I was surprised that nipples weren’t mentioned at all in the article since I’ve heard people use their presence as the main ’embryos initially start developing as females’ argument several times.

  2. Matt Kaiser says:

    You’re right that all embryos develop in the same way to begin with and that, under normal circumstances, without a hormone cue kicking in after a few weeks the female body plan forms. And it’s true that the fact that males have nipples shows this common developmental origin – I was going to put this in but I wasn’t sure I could include it without elaborating on the evolutionary reasons why males have nipples (i.e. why there is no selective pressure for men NOT to have nipples). The article I originally wrote was for a more lay audience and I didn’t want to derail the debate.

    I wanted to stay on track with the debate that I actually heard, which was about whether all early embryos are females (because of the pre-hormone ‘female’ template you mention) or are defined by their genetic make-up (XX or XY). I thought this threw up some interesting notions about gender distinction and definition, which is why I ended with the conclusion that it’s hard to define gender purely by development or by cytogenetics.

    Point taken though, male nipples would have reinforced the common developmental origin.

  3. Lesmond says:

    The thing is, before a critical developmental point when the genitals start to differentiate, the reproductive anatomy is neither male nor female. Female might be the “default”, so to speak, but early embryos are not anatomically “female” in any meaningful way. And they’re obviously not necessarily genetically female either.
    The “embryos start off female” thing is a total myth.

  4. Indeed, but I think the early formation of nipples has lead to that myth. I remember Robert Winston talking about it years ago when explaining foetal formation, that’s why I was surprised that it wasn’t mentioned here. Come on, who doesn’t like talking about nipples?

  5. Matt Kaiser says:

    Lesmond – that’s the conclusion I came to (i.e. gender-neutral) but also that even beyond that key developmental point, defining male and female in a binary way isn’t always straightforward.

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