No wonder the rhythm method does not work so well for birth control -- scientists in Canada said on Tuesday they had found women sometimes ovulate several times a single month. Their finding, if verified, would overturn the traditional wisdom that women produce an egg cell once a month. It would also help explain why "natural" methods of birth control, based on the idea that ovulation can be predicted, often fail. "We are literally going to have to re-write medical textbooks," said Dr. Roger Pierson, director of the Reproductive Biology Research Unit at the University of Saskatchewan, who led the...