THE reaction to Marissa Mayer's pregnancy has been polite, following the news she will have a boy three months after taking the helm of Yahoo.

Any criticism she may have anticipated from those doubting her ability to be a mother and a CEO at the same time, she stanched immediately with the pronouncement she would take just a few weeks' maternity leave. This invited criticism from some, who argued such a short leave could set a bad example for other working women. Mayer is in an impossible situation. However she manages her schedule, she will undoubtedly face the double standard so many women of all levels of employment face, that they are either neglecting their jobs or neglecting their kids.

While a complicated, intelligent debate is conducted over whether women can or cannot "have it all", on their own terms, society has not yet come to terms with having mothers in charge of multibillion-dollar corporations. Last year, a Silicon Valley investor said: "A pregnant founder/CEO is going to fail her company." As he contemplated an investment in a company run by a woman pregnant with twins, he thought: "How in the hell is this founder going to lead a team, build a company and change the world for these businesses carrying a kid around for the next few months and then caring for the kids after?"

In the broader business world, studies show women encounter most discrimination around motherhood. One Stanford study gave participants two CVs for a management consultant. Both were for women and both were identical, except one listed membership in a Parent Teacher Association. The woman with the child was 79% less likely to be hired, 100% less likely to be promoted and offered $11000 less in salary. Another study found highly motivated women were perceived to be so committed to work they must be bad mothers, which led to lower increases and fewer promotions.

Mayer has overcome at least one of these biases, winning over the board of Yahoo, which lauded her vast experience and qualifications. But her rise to executive level speaks to a related debate among professional women, two high-powered ones in particular.

Sheryl Sandberg, Facebook chief operating officer, advises women to find a supportive partner who is willing to share the work. She has told women it is OK to hire a nanny. But Sandberg was challenged by Anne-Marie Slaughter, former director of policy planning at the US state department. What she needed to change, she said, was the culture of the workplace: the idea that face-time and long hours, not efficiency, translated into good work. A chorus of women in the blogosphere chimed in that both had failed to address the vast majority of working mothers whose careers do not yield the kind of salary that can afford babysitters and had not paid enough attention to the role men play.

Already Mayer is being pushed to be a spokeswoman for low-income working women, and to set a positive example for other working mothers at Yahoo. And here she falls into another impossible situation, to be not only a good CEO and a good mother, but a good feminist, too. By virtue of being a pregnant CEO, she must take a stand on being a pregnant CEO, with critics waiting to pounce regardless of her opinion.

© 2012 The Financial Times Ltd