We discovered that when the amount of the gene fluctuates even slightly at any stage in the genetic blueprint, it changes the embryo from male to female

Dr Eric Vilain lead researcher

However, approximately one in 3,000 will be severe, and in many cases make it difficult for doctors to even tell whether the child is male or female.

In some cases, major corrective surgery can be required.

The cause of these "ambiguous genitalia" have always been a mystery to researchers, although there have been for some time suspicions that a selection of sex genes might be involved.

Comparison

The team, from the University of California at Los Angeles (UCLA) compared the genetic make-up of babies born with malformations to those born without.

They found that in many cases, the malformed babies had two copies of the WNT-4 instead of the normal one.

Dr Eric Vilain, who lead the research, said: "Our discovery offers insight into the genetic pathway of sex determination - what makes a man male and a woman female.

"These findings suggest that WNT-4 influences the sex determination pathway at each step of the way.

"We discovered that when the amount of the gene fluctuates even slightly at any stage in the genetic blueprint, it changes the embryo from male to female."

Prenatal diagnosis?

He believes that genetic testing could be used to improve diagnosis, perhaps before the child has even been born.

He even thinks that the defect can be "corrected" during foetal development - although this is only theoretical, and has never been attempted.

"If this proves true," he said, "we may be able to correct the defect in the womb and restore the embryo to its original male status.

"This may repair the genital malformations before the child is born."

The study was published in the American Journal of Human Genetics.