The Genetics and IVF Clinic, based in Maryland, has developed a technique which can separate out X and Y chromosomes.

It claims tests show a 92.9% success rate in determining the sex of a child, higher than any other technique.

Known as MicroSort, the process involves separating out sperm cells and increasing female X chromosomes or male Y chromosomes according to what sex of child the parent wants.

It is based on a technique used for gender selection in animals.

Scientists are able to separate out chromosomes using a special staining process which shows up the different types under ultra-violet light.

The chromosome-altered sperm is then inseminated in the woman.

Ethical debate

The scientists found that, by increasing X chromosomes by 85%, women were five or six times more likely to give birth to a female child.

Dr Edward Fugger: parents want to 'balance' their families

By increasing Y chromosomes by 65%, they say women are twice as likely to have a boy child.

In a recent study in Reproduction magazine, the scientists said 13 out of 14 babies born following treatment were female - as the parent had requested.

Dr Edward Fugger of the Institute said the technique could help couples who had several children of the same sex and wanted a child of a different sex.

"They are delighted and thrilled with the technique," he said.

But Dr Eric Widra, a US fertility specialist, said he was against the use of sex selection.