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.
How to chose your child's gender
By Esther Walker
Pre-implantation genetic diagnosis (PGD) using in vitro fertilisation (or IVF) is the most reliable way of influencing the sex of your baby.
During any IVF, eggs are removed from the mother and introduced, in the laboratory, to the sperm from the father.
If the couple want their child to be a particular gender, a cell is taken from each embryo three days after fertilisation, and the DNA and chromosomes are analysed. Only the embryos of the desired sex are subsequently planted back in the mother's womb.
It is, however, illegal in the UK to choose the gender of a child for social reasons, so PGD can only be carried out for medical reasons – for instance to avoid passing on genetic illnesses such as haemophilia (the disease where blood can't clot), which only affect boys.
Does it work? The only 100 per cent guaranteed way of choosing the sex of your baby
According to the study published yesterday, mothers who eat breakfast every day, preferably a bowl of cereal, are more likely to have a boy.
The headline-grabbing finding came about because, according to the research, women whose calorie intake is high at the time of conception (about 2,250 calories) have a 56 per cent chance of having a boy. Those whose intake is lower (around 1,750 calories) only have a 45 per cent chance of having a boy.
Elsewhere, there are many other theories about diet and its influence on gender, although many don't seem to be based on any hard science. For instance, to produce a boy, prospective mothers have been variously advised to eat lots of red meat (the redder the better), and salty snacks such as pretzels and crisps. Fathers should, allegedly, drink cola.
For a girl, both parents should eat a lot of fish and vegetables and gorge on chocolate, sweets and dairy products. (More colourful theories about the way diet supposedly affects gender can be found in the old wives' tales section below).
Does it work? A tiny effect at best, increasing the chances by a few per cent
Timing of intercourse
The idea that when you actually conceive can affect your baby's gender was first proposed seriously by the American doctor Landrum B Shettles, who wrote the bestselling book How to Choose the Sex of Your Baby.
The theory works, again, on the basis that male sperm swim faster than females. One of his theories was that if intercourse occurred very near to ovulation, you would get a boy – as the male sperm would beat the female in a short race to the egg.
Another theory based on the timing of intercourse was developed by another American, Dr Elizabeth Whelan.
She said that women would get a boy if intercourse was between four or six days before ovulation, when their basal temperature (the temperature one is when one wakes up) increases as part of the menstrual cycle.
If they wanted a girl they should aim to have intercourse two to three days before they ovulate. (Other theories about timing and gender can be found in the old wives' tales section).
Does it work? Nothing kills passion faster than trying to have sex to order, which would result in no baby
The effect of sexual position on baby gender is unproven, but most speculation about it is based around the theory that "male" sperm are small and fragile but quick, whereas "female" sperm are larger and tougher but slow.
The ancient Greeks believed that if a man had sex while lying on his right side, he was more likely to have a son.
Another long-standing theory has it that the "missionary" position is most likely to produce girl because the sperm ends up away from the cervix. This means that the sperm will have to fight through more acidic secretions to get to the womb – and as female sperm are tougher, they are more likely to get through than the males.
Having sex standing up, though, is said to produce boys, thanks to the theory that male sperm will be better at swimming, and so more likely than females to be able to swim against gravity.
Does it work? Unlikely, but it should be fun trying, nevertheless
Another scientific way of producing Janet and not John is artificial insemination after "sperm sorting" – literally separating "male" from "female" sperm.
There are two ways of doing this. The Ericsson technique works on the principle that Y-chromosome sperm (producing boys) swim faster than X-chromosome ones. The father's sperm is put through a sorting process, based on the speed it swims, and then only one particular kind is used for insemination.
The second method, microsorting, is based on the fact that X-chromosome sperm are slightly larger than the Y, carrying 2.8 per cent more DNA. Dye is added to the sperm sample, with the idea being that the larger sperm will identify themselves by soaking up more dye.
Does it work? It is better than nature but not guaranteed – it gives a nine-in-10 chance of producing a girl and a three-in-four chance of producing a boy
Old wives' tales
*In France in the 18th century it was thought that each testicle contained gender-specific sperm: so girls came from the left and boys came from the right. Men who wanted a boy were encouraged to tie off their left testicle during intercourse – or even to remove it.
*Girls are more likely to be conceived in the afternoon and on an even day of the month, while boys are more likely at night and on odd days.
*Female orgasms during sex are more likely to produce a boy, as the body produces alkaline chemicals after an orgasm, which are less likely to finish off the less hardy male sperm, which can then go on to beat the female sperm in the race to the egg.
*Men who drink coffee before they have sex are more likely to have a son, as the coffee perks up the male sperm but not the female.
*As mothers get older, they are more likely to have a boy.
*If you want a girl, sleep with a wooden spoon under your bed and a pink ribbon under your pillow.
*For a boy, only eat the ends of loaves of bread; for a girl, eat only the middles.
Do they work? If you believe these then you should watch out for the men in white coats
And... how to tell what the child's gender is when you are already "carrying"
You're said to be having a boy if...
* You're carrying the baby "low"
* You suspend a gold ring from a string over your pregnant belly and the string swings back and forth (rather than in a circle)
* The hair on your legs grows faster - supposedly due to increased testosterone in the body
* You're craving salty food
* You didn't suffer from morning sickness
* You have dry skin during pregnancy
You're said to be carrying a girl if...
* You're carrying the baby "high"
* You had morning sickness
* You're craving sweet things, such as sweets and fruit juice
* You combine your age at the time of conception with the number of the month you conceived and the resulting number is odd
* In the gold ring test, the string swings in a circle
* Your skin becomes very soft
(Of course, the best way to tell the sex of your baby is by ultrasound. And here's a tip: if you are asked: "Do you want to know the sex?", by medical staff, then you're not getting a girl. That's because ultrasound only provides certainty of the gender if the baby is male).