Can you choose your baby's sex?

Many parents long for a son or hanker after a daughter. As new research shows that strong women are more likely to give birth to boys, Roger Dobson sorts the science from the old wives' tales

For thousands of years, man has searched for ways of choosing the sex of a future child. Many pills, potions and positions have been tried down the decades, with little or no success.

It is now possible, with the help of fertility doctors using embryo selection, to choose the sex of a child, although the technique is not available in the UK. And parental demand for sex selection has never been greater.

But other methods of sex selection, albeit those that claim lower success rates than pre-implantation diagnosis, are being increasingly used, and research carried out among infertile women shows that four out of 10 would opt to choose to the sex of the child.

"We found that 41 per cent of patients would use sex selection if it were offered to them at no cost,'' says Dr Tarun Jain, an assistant professor of infertility at the University of Illinois.

"Sex selection is a topic that's almost taboo for doctors to talk about. Yet it's important to understand patient interest in sex selection for non-medical reasons, and address the ethical and social implications before the cat is out of the bag. Until now, there has been no data to indicate what the demand might be.''

But there are concerns that the effects of sex selection could alter the balance of male and female births.

The latest research in this area, at the University of Auckland, shows that dominant and confident women are more likely to have boys because the fertilised egg is exposed to higher female testosterone levels. Dr Valerie Grant, who led that research, says this work suggests that the sex of offspring in mammals may not be a matter of chance. Instead, sex allocation may be a finely tuned adaptation to the mother and her condition at or around the time of conception.

"Dominant females conceive significantly more sons. It could be that dominant women are specially adapted, both physically and psychologically, for conceiving and raising male infants, and non-dominant women female infants,'' she says. "If mothers of boys behave differently from mothers of girls, we need to know more about these differences before allowing people who are particularly suited to raising one sex to try their hand at the other.''

PRE-IMPLANTATION GENETIC DIAGNOSIS (PGD)

This is the gold standard for sex selection. It is accurate, but is only available for medical reasons in the UK, usually for sex-linked genetic disorders. PGD involves in-vitro fertilisation (IVF) and the removal of one cell from embryos and analysis of the chromosomes and DNA three days after fertilisation. Embryos of the desired sex can also be chosen. The downside is that it involves IVF, hormonal treatment, and the pregnancy rate for a cycle of PGD is around 20 per cent.

SPERM SORTING

Sperm carry an X chromosome or a Y chromosome, while all a woman's eggs carry an X chromosome. When an egg is fertilised by an X-bearing sperm, the child will be female, while a Y-bearing sperm will result in a boy. The aim of sperm sorting is to produce a sample with a higher than normal proportion of X or Y sperm, increasing the chance of conceiving a child of the sex wanted.

The Ericsson technique selects sperm on the basis that Y-chromosome sperm swim faster than X-chromosome sperm. For the man, it involves providing a sperm sample which is then put through selection processes to increase the proportion of female- or male-producing sperm. The woman has to undergo implantation through artificial insemination. It's claimed that 70 to 75 percent of couples have a baby of the desired sex. "The technique has attracted controversy as it is not clear how it works and attempts to replicate the results have not always been successful,'' says a report from the Parliamentary Office of Science and Technology on selection.

Another technique, the MicroSort method, is based on the X chromosome being substantially larger than the Y chromosome. As a result, its advocates say, sperm cells having an X chromosome will contain approximately 2.8 per cent more DNA than sperm cells having a Y chromosome. A dye is added to the sperm and the idea is that the bigger X-bearing sperm will soak up more of it and so be identifiable before being implanted. Success rates are reported to be 88 per cent for those X-bearing sperm, and 73 per cent for the Y-sperm.

"It is fascinating. It probably does work, and the results are very good. Although people need to realise it is not 100 per cent,'' says Stuart Lavery, consultant gynaecologist at Hammersmith and Queen Charlotte's hospitals in London. "The difficulty is that there are hypothetical risks associated with process and the DNA. The work so far is very encouraging and reassuring and around 350 babies have been born so far. Several patients have gone to America and gone through MicroSort.''

TIMING

A widely followed routine for maximising the likelihood of having a boy or girl, it was proposed more than 20 years ago by an American doctor, Landrum B Shettles, whose book, How to Choose the Sex of Your Baby, will be republished later this year. Claimed to be 75 per cent effective, one of the theories is that timing of intercourse is vital. According to the theory, the closer to ovulation you have sex, the greater the chance of having a boy because the Y-sperm is faster and gets to the egg first. It's said that having sex three days or more before ovulation helps the chances of having a girl.

Dr John Martin Young, a Texas sex selection expert, believed couples are more likely to have a boy if they avoid sex around ovulation and more likely to conceive a girl if they have intercourse during ovulation. He recommends women keep a chart of their monthly cycle. "If patients pay attention to the timing of their cycle their chances of conceiving the sex they'd like can be as high as two in three,'' he says.

The Whelan method, named after Dr Elizabeth Whelan, is claimed to be some 68 per cent effective for boys and 56 per cent for girls. It also centres on the timing of intercourse, but suggests that chemical changes that favour producing a boy occur earlier in a woman's cycle. If a boy is wanted, intercourse should be timed to occur four to six days before your basal body temperature - your temperature when you wake up - goes up. If you want a girl you should have sex two to three days before you ovulate.

DIET

Theories abound about the sex selection effects of diet, but there is little hard evidence. It's claimed thatcalcium-rich foods such as milk and cheese favour a girl, while red meat favours boys. One study on diet based on 281 couples found that those who kept to predetermined levels of calcium, magnesium, sodium and potassium before conception had a child of the sex they wanted in about 80 per cent of cases. A second study using a similar technique found that out of 47 births, only seven failed to produce the expected sex. How it works is unclear.

OVULATION PREDICTOR KITS

These tests, available from chemists, can be used, it's claimed, for finding the best time to have intercourse for producing a boy or girl. Some kits detect when the body releases a hormone just before ovulation. It's claimed that use of these kits will enable you to plan sex closer to ovulation with a greater chance of having a boy, but the kits are expensive and there's little hard research to support the idea.

TEMPERATURE

Sperm don't like heat, so it's suggested that heating will increase the likelihood of producing a girl. Supporters point to relatively high levels of daughters among airline pilots. The same theory led to the idea that men who wore tight underwear were more likely to have girls. So Y-fronts will favour girls, and boxers are more likely to produce boys. Scientists are sceptical.

Position, orgasms and coffee - theories from the wilder side

* Position: The ancient Greeks believed that men who had sex while lying on their right side were more likely to have a son. Missionary position is best for a girl, says Italian folklore, because it is more likely to place sperm away from the opening of the cervix. Shallower penetration means that sperm have further to travel to get to the cervix, passing through acidic secretions. Female sperm are made of tougher stuff and are therefore more likely to survive.

* Pain: In 18th century France, it was widely believed that if a man tied off his left testicle, a boy would be more likely. The theory was based around the mistaken idea that sperm from each testicle was sex-specific.

* Orgasm: Orgasms during sex are a good thing for having a boy, it has been claimed, as the body produces chemicals after orgasm that make the vaginal environment more alkaline, favouring Y-sperm. Women should reach orgasm before their partners to increase the chances of a boy.

* Coffee: If a man has a coffee before sex, the Y-sperm is more active and likely to result in a boy.

* Age: As a mother gets older, her chance of conceiving a boy increases.

* Dreams: Whatever sex of child a mother dreams of having, she will have the opposite.