Also says smoking and taking drugs can result in homosexuality

Claims stress in womb increases chance of baby growing up to be gay

Women who suffer stress during pregnancy increase the chance of their child being gay, according to a controversial new book.

The lifestyle of pregnant mothers affects the sexuality of their unborn child, altering their hormones and the formation of their brains, it claims.

Smoking or taking drugs can also result in a child who grows up to be gay, according to the book by Dutch neuroscientist Dick Swaab, which is yet another chapter in the debate about the origins and causes of homosexuality.

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Women who suffer stress during pregnancy increase the chance of their child being gay, according to a controversial new book (file picture). It claims a pregnant mother's lifestyle affects the sexuality of their child

Professor Swaab believes that a child’s sexuality is determined in the womb and cannot be altered, while others argue that it is affected by upbringing or ‘selected’ as a lifestyle choice.

In his book, We Are Our Brains, he writes: ‘Although it’s frequently assumed that development after birth also importantly affects our sexual orientation, there’s no proof of this whatsoever.

‘Children brought up by lesbians aren’t more likely to be homosexual. Nor is there any evidence at all for the misconception that homosexuality is a “lifestyle choice”.’

Professor Swaab says that the development of the brain during pregnancy is altered by the tiniest of chemical changes.

Professor Dick Swaab (left), author of the book We Are Our Brains (right), believes that a child's sexuality is determined in the womb and cannot be altered

As an example, he claims that a drug prescribed to two million mothers to combat miscarriages during the 1940s and 1950s increased the likelihood of bisexuality and homosexuality in their unborn children.

‘Pre-birth exposure to nicotine or amphetamines also increases the likelihood of lesbian daughters,’ he writes in the book.

‘Pregnant woman suffering from stress are also more likely to give birth to homosexual children, because their raised levels of the stress hormone cortisol affect the production of fetal sex hormones.’

And he claims: ‘The more older brothers a boy has, the greater the chance that he will be homosexual.

Professor Swaab says that the development of the brain during pregnancy is altered by the tiniest of chemical changes (file picture)

‘This is due to a mother’s immune response to male substances produced by boy babies in the womb, a response that becomes stronger with each pregnancy.’

Professor Swaab, based at the University of Amsterdam, has ignited controversy with his opinions before.

When he first explored differences in the brains of homosexual and heterosexual people in the 1980s there was a huge outcry from gay rights campaigners, who said his findings cast homosexuality as a ‘medical problem’.

But Professor Swaab, 69, says his view that sexuality is decided in the womb demolishes the argument, often made by ultra-conservative groups, that gay people can be ‘cured’.

Homosexuality is not the only subject of Professor Swaab’s wide-ranging book.

‘Pregnant woman suffering from stress are more likely to give birth to homosexual children' Professor Dick Swaab

His other controversial claims include the idea that annoying behaviour in teenagers may be evolution’s way of preventing incest, and that difficult births may cause schizophrenia, autism and anorexia.

Ben Summerskill, chief executive of gay rights organisation Stonewall, said: ‘There does not seem to be a shred of evidence to support the idea that a mother’s lifestyle changes a child’s sexuality.

‘Our feeling is that sexuality is probably is caused by genetic inclination, but until the evidence is there, nobody can say either way.’

Gay rights campaigner Peter Tatchell welcomed the professor’s ideas.