Can your diet really determine the sex of your child? Scientists this week declared that first-time mothers who consume more calories around the time of conception are more likely to give birth to boys. The theories surrounding gender selection are as outlandish as they are numerous. So what should we believe?
So it all comes down to the banana. The yellow fruit with the phallic shape – appropriate in this context – is the best dietary aid for intending mothers wanting a boy, according to research published yesterday.
In what they claim is the first scientific evidence that diet influences infant gender, researchers at the Universities of Exeter and Oxford have found that women who consume more calories around the time of conception, and, in particular, eat more bananas, are more likely to have sons. At the same time, those who skip breakfast (and breakfast seems to be key) are more likely to have daughters.
"We were able to confirm the old wives' tale that eating bananas, and so having a high potassium intake, was associated with having a boy," said Fiona Matthews, who led the study of 740 first-time mothers published in the strangely-named journal Proceedings of the Royal Society B.
But, she added, the source of extra calories was less important than the total amount consumed.
The differences are extremely slight. In the study, 56 per cent of women with a high-calorie intake had boys compared with 45 per cent with a low calorie intake. As a means to balance your family, this is only a marginal improvement on the 50:50 chance offered by nature. But the research has rekindled interest in the age-old "science" of sex selection.
Parents-to-be have, for millennia, longed to choose the sex of their child. Aristotle suggested that the ardour of the man at the moment of insemination determined the sex of the resulting infant. Like most theories since, it remains unsupported by the evidence.
In the 18th century, men desperate to produce sons – it was always sons – were advised to resort to the drastic measure of cutting off their left testicle, by a French anatomist who wrote under the name of Procope-Couteau. He claimed the surgery was no more painful than extracting a tooth.
For those reluctant to sacrifice their manhood, he suggested the woman should lie on her left side during intercourse so that "male eggs" could descend from her right ovary – an idea that derived from the ancient Greek belief that maleness and femaleness were determined by the body's right and left sides.
A hundred years later, the Victorians suggested that would-be parents who wanted boys should go on a strict diet because the male was the "starved sex" – which is exactly the reverse of yesterday's finding.
It was not until the beginning of the last century that the biology of sex selection – and the man's crucial role in it – was understood, which ushered in a new era of "scientific" attempts to influence the process.
At that point, biologists discovered that what distinguishes men and women is a single chromosome among the 46 that are found in every cell of the body. While men have one X and one Y chromosome, women have two X chromosomes.
The result is that men produce two types of sperm, one carrying the X chromosome and one the Y chromosome, while women only produce eggs with the X chromosome. The sex of the child they produce is determined by whether an X or a Y sperm fuses with the egg. If it is an X sperm then the resulting embryo will be XX (a girl) and if it is a Y sperm the embryo will be XY (a boy).
The challenge for those bent on controlling this process has been how to influence the production of X and Y sperm and ensure only one sort fertilises the egg.
A diet rich in milk has been said to favour daughters, while other dietary changes which increase the acidity of the vagina have been said to favour Y sperm, and thus sons. But, until yesterday, none of these dietary theories held up.
While couples experimented with altering the timing, and position, for sex, in the hope that it might influence the outcome, the science slowly advanced. By the 1990s, new techniques of "sperm sorting" were claimed to help those wanting a child of a particular sex. Gender-selection clinics opened in London and Birmingham offering the service and claimed to be doing brisk business.
There was scepticism from the start about whether their methods genuinely improved on the 50:50 chance offered by nature. One technique, based on the observation that X and Y sperm swim at different speeds, involved placing a sample of fresh sperm on top of a viscous liquid containing albumin (like that in egg-white). If the couple wanted a boy, the first sperm to swim to the bottom were collected and used to inseminate the woman.
Experts accepted the premise but were doubtful about the outcome. They said that, although Y sperm "wiggled more quickly", there was no statistical evidence that this method successfully separated them.
A second method, used at the Genetics and IVF Institute in Fairfax, Virginia, US, had more to commend it. This involved sorting sperm by laser, after staining them with a fluorescent dye, which enabled differences in the quantity of DNA carried by X and Y sperm to be detected.
Using the method, called flow cytometry, the clinic today claims to increase the proportion of X sperm in a sample to 88 per cent, giving couples who want a girl a near nine-in-10 chance of having one. Y sperm can be increased to the lower but still impressive level of 73 per cent, giving couples a near three-in-four chance of having a boy.
Even with this sophisticated technology, then, there is still a sizeable margin of error, and a risk of having the "wrong" sex of baby. The only way to be certain is to choose IVF. Once fertilisation of the egg with the sperm has taken place in the laboratory, doctors can distinguish male and female embryos and replace only ones of the required sex in the womb. Assuming the procedure results in the birth of a live baby, this is the only technique with a 100 per cent guarantee of success.
While the technology now exists to give parents their heart's desire, use of it is tightly regulated in the UK. There is revulsion at the idea of tampering with the balance of nature, allied to the disturbing prospect that we could end up with a disproportionate number of boys. Not perhaps as extreme as China, where the one-child policy has led to a massive dearth of girls, or India, where research suggests that girl infanticide is practised on a horrifying scale; but there is still an overwhelming view that choosing the sex of a baby for social reasons is beyond the pale.
As a result, for anything other than pressing medical reasons, sex selection has been banned in Britain since 1993. A year-long consultation by the Human Fertilisation and Embryology Authority (HFEA) in 2003 concluded that the ban should remain and, in fact, be tightened.
The consultation was triggered in part by the case of Alan and Louise Masterton, who had four sons and lost their only daughter, Nicole, aged three, in a fire in 1999. A fertility clinic in Nottingham had offered to help them recover from the tragedy by providing IVF with a female embryo to add a girl to the family as a replacement for their daughter. The HFEA refused permission for the procedure, and the couple later sought treatment in Italy.
Despite the tragic circumstances of that case, the result of the consultation showed that the public broadly backed the HFEA's position – 82 per cent of those questioned opposed sex selection for social reasons. The authority ruled that couples should not be permitted to choose the sex of a baby for family balancing and recommended a change in the law.
The legal move was necessary to close a loophole that had allowed gender-selection clinics in London and Birmingham to operate through the 1990s. The HFEA's regulatory powers only covered clinics offering IVF or using donor sperm. As the clinics used only the husband's or partner's sperm they fell outside its remit.
The loophole was closed in June 2007, when new EU regulations were introduced requiring any clinic dealing with sperm on a commercial basis to have a licence. In the UK, the new EU regulation will be enshrined in UK law in the Human Tissues and Embryos Bill that is currently going through Parliament.
The only exception to the restriction is sex selection to avoid serious gender-linked hereditary disorders, such as haemophilia and Duchenne muscular dystrophy, which affect only boys. Families affected by these disorders have always been permitted to apply for and receive IVF with pre-implantation genetic diagnosis to replace only embryos of the unaffected sex.
For all other couples dreaming of balancing their family with a boy or a girl, the options are limited. There may be no harm in experimenting with different sexual positions or timing of intercourse, altering the acidity of the vagina by douching or changing the diet. As a last resort, eating bananas may just help.
None of these, however, is likely to be as useful as simply remembering that children, of whatever sex, are gifts to be treasured, rather than a consumer commodity.
