Sheath that scalpel: the only way to be sure of an intersex baby's gender is to wait until you can ask them, researchers say.

Gender may have a lot more to do with the inside of your brain than the outside of your body. So when a child is born with indeterminate sex organs, doctors should hold off on genital surgery until the child has grown enough to identity as male or female.

That's the conclusion of researchers who presented their recommendation to a meeting of the American Association for the Advancement of Science in Washington, D.C., on February 18. "There is no one biological parameter that clearly defines sex," said Eric Vilain, MD, of the University of California, Los Angeles, whose research suggests gender is genetically hardwired before birth, regardless of which genitalia develop.

The recommendation bucks the current practice of performing immediate "assignment" surgery for most of the roughly one in 4,000 babies born intersex, many with both male and female traits. Because parents must pick a gender to name their child and fill out a birth certificate, specialists usually check chromosomes and hormones in order to assign male or female. But Vilain identified 54 genes that work differently in the brains of male and female mouse embryos just 10 days after conception, before sex hormones are ever produced.

William Reiner of the Oklahoma University Health Sciences Center told the meeting that he began seeing children who had been assigned to one sex as babies and a few years later began identifying themselves as the other. As young as 4 1/2 years old, the children would suddenly say, "I'm a boy," or pick a boy's name, Reiner said. His advice to parents: Think hard before agreeing to surgery for an intersex baby. Dealing with the social trauma of switching gender later is enough without the issue of surgery that can't be reversed.