Not having it all is so last week. Now here comes Marissa Mayer, 37, who in a single day showed up for her new job running Yahoo and announced that she’s expecting her first child in October.

Of course, Yahoo officials knew Mayer was pregnant when they hired her. “They showed their evolved thinking,” she told Fortune magazine. Which makes me want to call for the opposite of a boycott.

The decision to hire Mayer – Google’s first female engineer, who reportedly had been running about a quarter of the company – not only sends a great message to women (and about Yahoo) but may have a trickle-down effect, at a minimum inside Mayer’s new shop.

How big a deal is this? According to TechCrunch, this is a first for a chief executive of a publicly traded Fortune 500 company. And here’s how much things have changed in the 16 years since my twins were born: Not long after I went back to the office – initially working three days a week, although that quickly crept up to four and then five – one of my colleagues informed me that she and others in the office thought that I should no longer be “allowed” to write for Page One; that it wouldn’t be fair to reporters without children, she said, who had not been able to take a leave – and who were expected to work seven days a week.

“She the People” writer Diana Reese remembers those days, too – and recalls interviewing for a journalism job when she was single and four months pregnant in 1991: “I was skinny then and not showing much, so I safety-pinned my suit skirt closed, made sure my jacket hid my little baby bump, and lied through my teeth. Everyone I knew cautioned me not to say a word about being pregnant. The general consensus was that they would never even interview me if they knew.”

The Washington Post’s Nikita Stewart says she, too, hid the fact that she was pregnant when she interviewed for a reporting job at the Star-Ledger in Newark 13 years ago, revealing it only after the paper made her a firm offer, “because I feared they would never give me a chance” otherwise.

She got the job, showed up right after her maternity leave from her previous employer – “that was also a trick” – and has been kicking journalistic derriere ever since.

But we shouldn’t have to pull “tricks” to have kids and also support them doing work we’re good at.

Even in this economy, it isn’t only top executives who no longer feel the need to pull any fast ones, thank goodness. Nothing makes me happier than looking around the Post newsroom, where there are many other women, nine of whom are expecting. And I can’t imagine anybody telling any of them that they’d better not come back thinking they have a right to jump right in without being made to feel bad.

How “having it all” ever became the impossible dream I’m not sure, but if all of our work/life challenges were behind us, Anne-Marie Slaughter’s recent Atlantic cover piece, “Why Women Still Can’t Have It All,” would not have become the best-read story in the magazine’s history. Slaughter, a former director of policy planning for the State Department, left after two years to spend more time with her teens and return to her former teaching job at Princeton – hardly a nothing post.

On Monday night on “The Colbert Report,” Slaughter stipulated that in trying to have maybe not “it all,” but important bits of it, it makes a big darn difference whether you’re wealthy and have hot- and cold-running nannies. (You bet it does, and in what area of life is that not true?)

Obviously, it’s because Mayer is so highly regarded – as my Post colleague Roxanne Roberts said, Yahoo needed her more, a lot more, than she needed it – that she can get a deal like this, one that makes a moot point of the glass ceiling and knocks out the wall where the nursery adjoining her office will be.

A pregnant friend of mine worries that Mayer’s stated intention to take only a few weeks off after giving birth – and even then, she’ll be working from home – might suggest that women who take a real maternity leave are slackers.

But if she can turn around Yahoo while breast-feeding, she will have pulled off what a succession of chief executives before her couldn’t manage – and if she doesn’t, I dare to hope they won’t blame it on her baby.