In each, male and female embryos are initially equipped to go either way. A human embryo develops the foundations for both a female set of ovaries and a male set of testes in the first weeks. A molecular chain of events orders one set to disintegrate at about eight weeks. At about nine weeks the embryo starts to develop external genitalia that match the internal sex organs. At Stanford Medical Center, a baby's genitalia and internal organs aren't consistent with one sex or the other as many as 20 births a year - and doctors do not know whether to announce the arrival of a boy or a girl.
Clinicians look first to the internal anatomy, then to the chromosomes for clues.
In the most difficult cases, a team of specialists must work together to reach a sex assignment best for the child and to explain to parents that sex identity isn't solely a matter of Xs and Ys. Using hormones and delicate surgery, doctors try to create a sex the child will be able to live with. After a decade of research, surgeons at Stanford Medical Center can perform complicated reconstructions in just one operation instead of several. Dr. Linda Shortliffe, chief of pediatric urology and chair of the department of urology, can reduce a masculinized girl's clitoris, separate a combined vagina and urethra, and create or reform the vagina. Or she may reconstruct a boy's entire urethra, shifting the opening from below the scrotum to the top of the penis and straightening the penis.