Equal rites / Pratchett, Terry

The last thing the wizard Drum Billet did before Death laid a bony hand on his shoulder, was to pass on his staff of power to the eighth son of an eighth son. Unfortunately for his colleagues in the chauvinistic (not to say misogynistic) world of magic, he failed to check on the new-born baby's sex.

A dying wizard attempts to pass his powers to an eighth son of an eighth son who is in the process of being born, not realizing until it is too late that the newborn is a girl.