No sooner was Marissa Mayer named chief executive of Yahoo! than she announced she's pregnant. In doing so, she ramped up a lively ongoing debate in the United States about working mums.

Mayer, 37, a Silicon Valley high-flyer, who quit the top ranks of Google to take the helm of its struggling rival, took to Twitter to say she's expecting "a new baby boy" in October with partner Zack Bogue, a venture capitalist.

"I like to stay in the rhythm of things," she later told Fortune magazine.

"My maternity leave will be a few weeks long, and I'll work through it," she added, as if to reassure Yahoo! shareholders that motherhood won't get in the way as she settles into her new corner office.

Ensuing cheers in cyberspace drowned out the fact that Yahoo! ­­- which last week reported a 4 per cent drop in quarterly profits to US$226.6m (S$285.99m) - is struggling in the dog-eat-dog world of Internet technology.

"Lets (sic) hope she inspires corps (corporations) to create better options for all working moms," tweeted actress Mia Farrow, a mother of 13 biological and adopted children who famously carried Satan's child in Rosemary's Baby.

"I applaud @marissamayer. Bravo! & congrats re baby, best news of all. She'll inspire countless women in tech & beyond," echoed Princeton University professor and mother-of-two Anne-Marie Slaughter, also on Twitter.

Slaughter, who quit a high-powered US State Department job to spend more time with her family, stirred up a furore this summer with an essay in The Atlantic magazine, asking if it was truly possible for working mothers "to have it all".

But Diana Limongi, a young mother in New York who blogs about motherhood issues under the name LadydeeLG (ladydeelg.tumblr.com), had more sobering words to offer Mayer, based on experience.

"To claim that she will be 'in the rhythm of her work' ... is not acknowledging a reality we have to accept and come to terms with: Babies need care, mums need time to heal and to be with their babies and that is OK," she wrote.

Mayer is Yahoo!'s fifth boss in the past year, but not the first with kids. That honour went to Carol Bartz, a mother of three who was sacked in September last year after failing to turn the flagging company around.

Just 19 companies on the Fortune 500 roster of top US corporations are run by women - "a new record," the magazine said. Many of those women are mothers, but it's unheard of for a new CEO to get hired on threshold of giving birth.

Out of the 34 million mothers in the United States with children under 18, nearly half - 47 per cent - work full-time or part-time, according to the US Census Bureau.

Few can dream of the perks - like nannies, day care or the nifty Yahoo!-branded layette set she got last Tuesday from her new colleagues - that Mayer will be able to afford on her expected multi-million-dollar salary.

By US federal law, American women are entitled to 12 weeks of unpaid maternity leave - and that's only if they're in a workplace with more than 50 employees, and they've been on the job for at least a year.

Indeed, the United States remains the only Western country without paid maternity leave.

Moreover, the past 10 years has seen a 35 per cent increase in the number of complaints to the Equal Opportunity Employment Commission arising from discrimination on the basis of pregnancy, Sarah Crawford, director of workplace fairness at the US National Partnership for Women and Families, said.

"We hope that Yahoo!, with (Mayer) at the helm, will be a pacesetter for family-friendly policies."

Maria Bailey, founder of Bluesuitmom.com, a website for working mothers, said Mayer should have no problem juggling the work-life balance so long as she commits some serious quality time to her child.

"I would say, as someone who is a former corporate mum as well, that you can do it all ... but it's extremely important to be present with your children, when you are with your children," she said.

"You have to put boundaries around your time," said Bailey, who regretted that American society still looks at mothers in the boardroom - and not just pregnant ones - as something of a novelty.