"Another piece of good news today," tweeted the expectant mom, announcing to her online followers that she and her husband are awaiting a baby boy.

But this wasn't just any excited mom-to-be. This was 37-year-old Marissa Mayer, the newly named CEO of Yahoo — obviously a huge achievement for anyone, but especially for a woman in the male-dominated tech industry. And she was about six months pregnant, to boot.

Exciting news — especially for Mayer and her husband, of course — but did it mean something for the rest of us, too? Was it a watershed moment in the perennial debate over whether women can "have it all," with the pendulum swinging happily in the positive direction?

Or was it, as some claimed in the inevitable back-and-forth on Twitter, actually a development that would increase pressure on other working moms, who might not have nearly the resources that Mayer does, in terms of wealth, power, talent and flexibility on the job?

Or was it even sexist to raise the question at all? Would anyone be saying anything if the new Yahoo CEO were an expectant father? No, went a frequent online thread: No one would even pay attention to that.

What was clear was that Mayer's situation as a pregnant CEO of a Fortune 500 company is not only rare, but probably unique. She becomes only the 20th current female CEO of a Fortune 500 company, according to Catalyst, an organization that tracks women's advancement in the workplace. If it sounds like a lot, it's not; that's only four percent of Fortune 500 chiefs.

There is little or no research tracking whether any have been pregnant while in that job, but a look at the other current female Fortune 500 CEOs shows that the vast majority are well into their 50s, and thus presumably well out of maternity-leave territory.

Mayer, who left Google to take the new job, wasn't speaking — tied up with her first-day responsibilities Tuesday at Yahoo, she declined interview requests, including one from The Associated Press. But on Monday, she told Fortune magazine that the Yahoo board "showed their evolved thinking" by hiring a pregnant chief executive, and that she plans to take only a few weeks maternity leave — during which she would work throughout.

That raised a few eyebrows among some who suspected it might not be as easy as the first-time mom thinks.

"She will also, I am betting, not power through quite as single-mindedly on her maternity leave as she thinks she will," wrote Lisa Belkin on her Huffington Post blog.

Many speculated that, like other working moms, Mayer would find her attentions and energies divided well beyond maternity leave.

"Anyone can have it all," said Julie Marrs, a sales administrator in Conroe, Texas, "but maybe not be as successful at everything as one hopes." Marrs, a mother of two boys who works full-time, said she has learned the hard way that something always gets sacrificed.

"There are times that I am so mentally drained when I get home from work that I definitely do not spend the time I should with my kids," she said in an email message. "Whether it be working on homework, reading books, playing a game or simply talking about their day. I try my best, but realize that to 'have it all,' something will be sacrificed. It could be takeout four nights during the week instead of a hot, home-cooked meal. Or it could be hearing your child read their first story book."

While most online chatter about Mayer was full of praise for both her and Yahoo and sometimes saying "You CAN have it all!," there were those who said Mayer was perhaps not the best example to prove such a thesis.

One of them was Anne-Marie Slaughter, whose online lament last month on The Atlantic's website — "Why Women Still Can't Have it All" — unleashed a furious debate, showing that while the question might be a perennial one, it hasn't lost any punch.

"Well, I think it's fabulous news," Slaughter said of Mayer's appointment and pregnancy in a telephone interview. But, she suggested, Mayer's situation — with her wealth, prominence and power — has little concrete relevance to the lives of ordinary women. (Mayer's personal wealth has been estimated in the hundreds of millions of dollars, largely due to Google stock that she owns.)

"We all applaud her," Slaughter said. "But she's superhuman, rich, and in charge. She isn't really a realistic role model for hundreds of thousands of women who are trying to figure out how you make it to the top AND have a family at the same time."

She also noted that Mayer had arrived at her position of prominence by following the route many working women in Slaughter's generation — she is 53 — had followed, including Slaughter herself: waiting until after 35 to have a child, so as to first establish yourself professionally.

"It's a risky route, because of that biological clock," she noted.

While many applauded Mayer's achievement, a few were disappointed that the big news of her pregnancy was, well, big news.

"It is great that Marissa Mayer is pregnant," tweeted writer Rebecca Traister. "But the intensity of reaction is slightly depressing. Kind of as if they'd hired a yeti."

Catalyst, the organization that tracks women in the workplace, also noted that it hopes one day, "there will no longer be a need to count" the women CEOs in the Fortune 500. Others called Mayer a role model, but then said they wish that she didn't have to be.

Jen Singer, a mommy blogger based in New Jersey, had another concern. Mayer, she said, was certainly going to have more flexibility, being at the top. But, she added, "She also has more responsibility."

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"When most women announce their pregnancies, it doesn't affect the value of the company's stock. Just like Steve Jobs for a time insisted his health was fine to protect Apple, Mayer must look like she can have the baby and handle the company at the same time.

"I just hope she doesn't screw it up for the rest of us," Singer said. "Because whether she likes it or not, she is now the poster child for working mothers."