Levels of stress are also a determining factor in whether a child turns out to be gay, writes Dick Swaab, Professor of neurobiology at Amsterdam University.
In excerpts of his book We Are Our Brains published in The Sunday Times, he suggests lifestyle choices adopted by mothers-to-be can radically affect everything from a child’s sexuality to IQ.
“Pregnant women suffering from stress are also more likely to have homosexual children of both genders because their raised level of the stress hormone cortisol affects the production of foetal sex hormones. “
The book also claims the more older brothers a boy has, the greater the chance he will be gay.
It adds some anti-depressants and alcohol – even a glass of wine a day – can result in effects such as a lower IQ and hyperactivity, stating; “There is no safe level.”
A blurb from the book, which was released in hardback on 16 January, states: “… nearly everything about us – from our sexual orientation to our religious proclivities is present in our neuronal circuits before we are even born.”
Richard Lane, media manager at Stonewall, told HuffPost UK such scientific findings "seem to come along in groups every so often purporting to 'prove' the same thing. We've yet to be convinced that there's any real hard evidence behind the studies."
He points out that the studies at least say that being gay or bisexual is something inherent to an individual, rather than a lifestyle choice, as some opponents of equality continue to suggest.
He added: "Our concern with this study is that it might encourage expectant mothers to take up smoking in the hope that they'll end up with a gay child."
Gay rights campaigner Peter Tatchell welcomed the claims, telling the Daily Mail: “It is a slapdown for religious and political homophobes. If being gay is mostly or wholly determined by biological factors prior to birth, it is immoral to condemn or discriminate against lesbians and gay men.”