Pink or blue brains in the womb?

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This is an excerpt from the Introduction to The Women’s Brain Book. The neuroscience of health, hormones and happiness. Also published in the UK and Ireland as Demystifying the Female Brain. A neuroscientist explores health, hormones and happiness. 


Mother Nature is selfish. Her one and only goal is for us to have sex to make babies. To ensure dating and mating occurs, the parts of our brain that control reproduction, in particular, the hypothalamus, become ‘masculinised’ or ‘feminised’ to match our male or female gonads.

Sex hormones organise the prenatal brain

Hormones influence how these reproductive brain circuits grow and respond. Prenatal life is the first of two life phases when the brain is super sensitive to sex hormones.

We call this early period of hormone exposure the organisational period, as it organises or programs the brain to respond to hormones in adulthood. Many sex differences that do exist are developmentally organised and then activated, or revealed, by the action of hormones at puberty.

Is the female brain the ‘default’ option?

During the prenatal period, the dominating influence is that of testosterone produced by foetal testes. Testosterone helps to ensure that the reproductive regions of the brain in males becomes masculinised.

At the same time, and in the absence of testosterone, brain regions involved in reproductive behaviours in females become feminised.

In other words, in the absence of a Y chromosome, the default developmental option is for the foetus to become a female.

‘Default’ is a term some people find a little dismissive.

To help unravel the complexities of male versus female prenatal brain development, I called Margaret McCarthy, a professor of neuroscience who studies the effects of hormones on brain development at the University of Maryland School of Medicine. McCarthy is one of the pioneers in the field and conducted some of the first studies on how sex hormones organise the developing brain.

“Default does not mean passive!”

McCarthy said to me in a tone of voice that had me quickly convinced she’s made that statement more than once.

“Try using ‘The developing mammalian brain is destined for a female phenotype’, instead!” she said

In the absence of a Y chromosome, all embryos are destined to become female.

What is the role of oestrogen in female brain development?

You might be wondering, if testosterone ‘masculinises’ the unborn baby boy, what role does oestrogen from foetal ovaries play in ‘feminising’ the unborn baby girl?

Believe it or not, foetal oestrogen plays no role at all. Female embryos do not require ovarian hormones to become feminised (remember, they’re destined to become female). Oestrogen’s role in the developing female brain is thus a by-product of its absence, rather than presence.

Unborn babies’ brains are also shielded from the influence of maternal oestrogens (made by their mother and the placenta) by a molecule called alpha-fetoprotein, which is made in the liver of an unborn baby. It binds to oestrogen in the bloodstream and prevents maternal oestrogen crossing into the baby’s brain.

Curiously, oestrogen is involved in organising the architecture of the male brain. Testosterone easily passes into unborn baby boy’s brains where it is converted to oestradiol by an enzyme called aromatase. It is now well established the ‘female’ hormone oestradiol is responsible for ‘masculinising’ the male brain in utero.

Mother Nature is selfish. And she also has a sense of humour.


This is an excerpt from Chapter One of The Women’s Brain Book. The neuroscience of health, hormones and happiness. Later in the chapter, I explain why supposed sex differences in achievement in maths, interest in technology subjects or ability in the sciences cannot be attributed to the presence (or absence) of testosterone in the womb. In sum: the role of hormones can be enhanced or entirely eliminated by how girls and boys are raised.

Find out how to get your copy here

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3 Comments

  1. Kennedy on March 7, 2021 at 2:20 pm

    Hey Dr Sarah, I stumbled across you on Instagram and have been scrolling through your blog. It’s very interesting, informative and easy to grasp for us non-brain science folks.

    I have a question about in-utero hormones not being linked to interest and achievement gaps between the sexes. Is it a case of hormones and genes not playing any part at all or that hormones and genes are not the full story, nurture has a lot to say about it too.?

  2. Lourdes Goodwin on January 18, 2024 at 11:19 am

    Hi, I found the use of terms that represent social constructs like Mother Earth, selfish or even feminisation and masculinisation in a physiological based blog counterproductive. The contribute to sustain biases and misinterpretations.. I suggest add clear explanations or what is meant by using them or avoid them altogether.

    I will be more interested in reading the exact changes in the brain that are related to hormones and their implications.

    • Sarah McKay on January 23, 2024 at 3:52 pm

      Thanks sharing your thoughts, Lourdes. I appreciate your perspective on using terms like ‘Mother Nature’ and I understand your concern about the potential for sustaining biases and misinterpretations. The terms you mentioned are often used metaphorically to add a layer of narrative or to personify complex scientific processes in a way that might be more relatable to a general audience. However, I see how they could distract from the physiological focus of the blog.

      In my books, which delve deeper into the nuances of how hormones influence brain changes, I aim to strike a balance between rigorous scientific explanation and accessible language. It’s important to me that my readers feel informed and comfortable with the content, and I’m thankful you’ve highlighted an area for improvement. If you’re interested in the detailed science of hormonal effects on the brain, I believe you would find the in-depth discussions in my books very insightful.

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