Concern over baby sex 'guarantee'

There are fears that the technology could be abused by people wanting to determine their child's sex for purely social reasons.

And eminent British fertility expert Lord Winston warned parents against using the Fairfax procedure, which he said could cause damage to the unborn child.

The new technique involves separating out sperm that will produce a female embryo. This can be done because they carry more genetic material than those that will produce males.

Other gender selection techniques are available, but this is more accurate, and does not rely on discarding already fertilised embryos.

US researchers claim their method is accurate on nine out of 10 occasions, and say that their technique could help couples to avoid passing on genetic illnesses, which usually affect only boys.

It is the contents of the sperm that decide whether a baby will be male or female. A sperm that carries a Y chromosome will produce a boy, while one that carries an X chromosome will produce a girl.

Heavy X

The key difference between the two is that the X chromosome is substantially larger than the Y chromosome - carrying 2.8% more DNA.

The MicroSort technique, developed at the Genetics and IVF Institute in Fairfax, Virginia, exploits this difference.

Dr Harvey Stern, presenting the team's work at the European Society of Human Reproduction and Embryology congress in Lausanne on Wednesday, said the device was currently undergoing a clinical trial, but experiments so far had been highly successful.

He said: "We were able to be unambiguous in assigning gender in 90% of the 284 embryos.

"Of these, 92% were female and 8% were male. It substantially increases the chance of a couple having a child of a particular gender."

Double chance

They say it could help to prevent babies being born with many devastating genetic disorders, such as haemophilia and muscular dystrophy that are caused by defects on the X chromosome.

However, Lord Winston told the BBC: "There is a question that they might be letting themselves in for more than they bargained for, so far as the risk to the unborn child is concerned.

"The concern that most of us have is that the procedure itself might change the nature of the DNA and give a long-term risk of damage to the children born.

"Any change in the DNA most of the time will be harmless, but where it affects a gene there is a risk that a gene might be changed."

Lord Winston also claimed that the technology would be of little use in screening for sex-related disorders.

Possible abuse

Professor Ian Craft, head of the London Fertility Clinic, said similar work was being carried out in the UK- but not with such impressive results.

He admitted that there was a concern that the process could be abused for social reasons, and said the law would have to be changed if new controls were to be introduced.

"We have to face facts, the public do require this sometimes, whether it is for inheritance reasons, or balancing their families. They will go to any lengths to have a child.

"Ultimately we have to wonder whether will you ever have sex selection kits available at your chemist, whereby you add something to your semen sample and get an X chromosome or a Y."