NEW YORK - Yahoo CEO Marissa Mayer is promising to be away from the job for just a "limited time" when she gives birth to twins later this year, but the development only adds to the uncertainty some investors have long had about Yahoo.

While a joyful personal event, it shows how a big-name CEO can rattle a company when disclosing significant medical or other personal news. Think Martha Stewart heading to jail or JPMorgan's Jamie Dimon's cancer diagnosis.

"On the one hand the CEO wants to prove to investors they're indispensable," said Jack Ablin, chief investment officer at BMO Private Bank. "On the other hand, they need to present a picture that the company can survive without them for a while. It's a balancing act."

Mayer, 40, said Tuesday she and her husband are expecting twin girls in December and she said she plans to work through the pregnancy much like she did three years ago, when she had a baby three months after taking the helm at Yahoo.

At that time, Mayer's limited time off set off a nationwide debate about corporate culture and working mothers. A few months later, the company expanded its parental leave policy for both mothers and fathers and added other perks for new parents.

Now, Yahoo is in a very different place. Investors already have been questioning Mayer's strategy because Yahoo's ad sales have barely budged during her tenure, even though marketers have steadily increasing the amount of money they are spending on the Internet. Most of those dollars, though, have been flowing to Google and Facebook.

"It has to be frustrating if you are an investor, and it's not about the kid she had before or the kids that are coming," said Rosenblatt Securities analyst Martin Pyykkonen. "It's because she hasn't managed the advertising business very well."

Yahoo's stock price was little moved on Tuesday. Its shares ended the day down 64 cents at $31.60, amid a generalized sell-off on Wall Street. The stock has tumbled 37 percent since the beginning of the year.