Mary Barra made corporate history seven months ago when she became the first female CEO of a major global carmaker. But her rise to success is still an anomaly among the very largest companies.

That’s because unlike Barra, who’d been in charge of General Motors Co.‘s product development for two years before her appointment, a majority of top-ranked women in the Standard & Poor’s 500 Index aren’t in the kinds of operational jobs that lead to a chief executive office. Rather, 55 percent of them are finance chiefs, top lawyers or heads of human resources, according to data compiled by Bloomberg.

About 94 percent of S&P 500 CEOs held top operations positions immediately before ascending to the top job, and the relative scarcity of women overseeing product lines or entire businesses risks slowing their advance to the very top. The data show that the next generation of female executives is poorly positioned to capitalize on recent progress at a time companies from Google Inc. to Apple Inc. are laying bare their lack of diversity to help raise the number of women and minorities in the workforce.

Read more: Crain’s Detroit Business