Battle of the sexes

Have a burger and chips before getting pregnant and you're more likely to have a baby boy whereas a girl is more likely if you eat chocolate or ice cream. It may sound about as convincing as puppy-dogs' tails, but this is the latest cutting-edge science as reported in New Scientist.

Researchers at the University of Pretoria in South Africa found that mice given drugs that reduced their blood-sugar levels produced more female than male pups. And the finding fits with traditional wisdom that mothers should feed on red meat and salty snacks if they want a boy and chocolates and sweets if they want a girl, according to lead researcher, Professor Elisa Cameron.

The team used a steroid called dexamathasone that inhibits the transport of glucose into the blood to study the impact of raised or lowered blood sugar level. "It is very interesting that meat raises blood sugar levels for a sustained period of time while sugar-based snacks are asso-ciated with a slump in blood glucose," she explains.

Cameron's study adds to the evidence that a baby's gender is not simply down to whether the father delivers an X or a Y chromosome. The environment of the womb, it is claimed, can be manipulated to favour either male or female sperm.

Over the last half decade, animal research has come up with the same kind of results. Rats fed a low protein diet prior to conception in a Japanese study had more females in the litter, while American researchers reported that a diet rich in animal fat led to more male mice pups.

For Professor Richard Sharpe, head of the Medical Research Council's Human Reproductive Sciences Unit at Edinburgh University, however, the findings are not necessarily good news. "There is a global trend that more girls are being born than boys and the number of girls are increasing," he says. "It's a fractional increase. But all the evidence suggests that the underlying trend is an increase in physiological stress in women that causes the low blood sugar levels that is being reported in the New Scientist."

A major cause of this stress appears to be pollution, particularly from the chemical dioxin. One of the biggest leaks of the chemical, from a factory in the Italian town of Seveso in 1976, had a dramatic impact on the sex ratio of babies born in the area: in the seven years after the accident, births totalled 46 girls and only 28 boys.

A similar impact was reported in October (2007) from airborne dioxins: a Canadian study published in the journal of the American Chemical Society found that the sex ratio of newborns was 54 females to 46 males in a 25-mile radius of sources of dioxin pollution compared with the normal 51 boys born for every 48 girls.

Such an effect makes sense from an evolutionary viewpoint, says Michel Odent, director of the Primal Health Research Centre (www.birthworks.org). "In a population facing an adverse situation where survival is paramount, it makes good evolutionary sense to increase the number of females and to eliminate the weakest males. In that way, the healthiest males survive."

It also makes biological sense, says Professor Sharpe. Male Y chromosome sperm are more fragile than the X chromosome and the male of the species is more vulnerable both at the point of fertilisation and for the difficult first few weeks of life in the womb.

"We know that the process of masculinisation is a cascade of events over a period of seven weeks and culminates in the switching on of a key gene on the Y chromosome at seven weeks," says Professor Sharpe. Until that time, he explains, both sexes have "indifferent" gonads and "the female gender is the blueprint".

As well as pollution, the weight of the mother may also play a part, he says. "Women's weight is increasing at the same time as the ratio of female babies. Women with an above average BMI [body mass index] are more prone to gestational diabetes and have lower blood sugar levels and this seems to explain why they have a higher chance of having a girl baby. It could even explain why some researchers claim that single mothers are more likely to have girls, as they may be subject to more physiological stress," he says.

Thousands of couples are yearning for a baby of a particular sex, but Professor Sharpe warns that "messing about" with diet is not a good preparation for conception. "There are a few simple steps that are worth taking to prevent harm to the baby stopping smoking and drinking, eating a healthy diet, cutting out irritants including cosmetics. That's the responsible way to prepare for parenthood."

How to choose: the theories

* Pre-implantation genetic diagnosis The gold standard method is the only accurate method of gender selection and the one that is used when gender selection is carried out for medical reasons in the UK.

* Sperm sorting Known as the Ericcson technique, it's based on the idea that Y chromosome sperm can be separated from X chromosome sperm because they swim faster. "In my view, companies that claim to be able to do this are charlatans," says Professor Richard Sharpe.

* Timing Have sex three or more days before ovulation and you're more likely to have a girl, according to a routine first described by Dr Landrum B Shettles more than two decades ago. Sex closer to ovulation increases the chances of a boy as Y chromosome. Websites still encourage this and a new edition of Shettles' book, How to Choose the Sex of Your Baby, was recently published but there is no proof it works.

* Acid or alkaline This popular theory promotes the view that X chromosome sperm is better able to survive in a harsher acidic environ-ment in the female reproductive tract while the Y-chromosome is comfortable in alkaline. Go online to buy pH (measure of acidity or alkalinity) testing kits as well as acid and alkaline douches. However, experts say there is little chance of this method working.