For the first time, scientists have been able to detect small but significant physical differences between human sperm carrying the X-chromosome - which result in girls - and those with the Y-chromosome, which lead to boys.
The scientists suggest the difference between the two types of sperm could be used to select the sex of babies conceived by artificial insemination.
Colin Matthews, professor of reproductive medicine at the University of Adelaide, Australia, and his colleague Ke-hui Cui report in the journal Nature how they analysed 217 individual sperm, 106 with the Y chromosome and 111 with the X chromosome.
They took photographs of the sperm, magnified 30 times. 'Statistical analysis showed the length, perimeter and area of the heads, and the length of the neck and tail in X sperm are significantly larger than those in the Y sperm . . . (this shows) for the first time that human X sperm are statistically larger and longer than Y sperm,' they say.
Professor Matthews told the Independent: 'It may be possible in future to use this to perhaps separate male and female sperm.' He said this could benefit parents carrying defective genes for sex-linked disorders which affect one sex but not the other, for example haemophilia and Duchenne's muscular dystrophy in boys.
There will also be pressure, however, from parents who want to select one particular sex for social rather than medical reasons, he said. 'The first step is to get a technique that works and then to translate the ethics of using it into the culture you are in,' Professor Matthews said.
The Human Fertilisation and Embryology Authority has called for sex-selection techniques not to be used for social reasons, but recognised that advances in technology and changes in public attitudes will mean it will have to review its position in the future.
The authority became concerned about sex selection following the opening earlier this year of the London Gender Clinic, which claims to be able to separate male and female sperm by exploiting alleged differences in the speed at which each swim in a viscous solution of albumin protein.
Y sperm are alleged to move faster than X sperm.
However, many specialists in human reproduction have cast doubt on the claims. Several groups of scientists have been unable to reproduce the effect, devised in the 1970s by an American scientist, Roland Ericsson, from a company called Gametrics in Montana.
Professor Chris Polge, scientific director of Mastercalf, the company that has successfully separated cattle sperm by a different process, said he is sceptical of the London clinic's technique.
The London clinic said it had achieved a success rate of 77 per cent for boys and 70 per cent for girls. But Professor Robert Winston of Cambridge University, a pioneer of test-tube fertilisation, has called for independent verification of the clinic's claims.Reuse content