“It’s like wildfire the minute you don’t put the cocktail glass to your mouth,” Frere said. “My husband’s family is the first one to start gossiping about who’s not doing what.”

As the world knows, Kate and Will, the Duchess and Duke of Cambridge, were trying to keep their pregnancy private, in their case amid speculation that grew ever more frenzied as they toasted with water, not wine, at a Singapore gala in September, and as Prince William accepted a baby outfit from a well-wisher. The couple kept the public guessing until Dec. 3, when severe morning sickness sent Kate to the hospital, and the Palace was forced to issue a statement weeks before the expectant parents had planned.

But even women who are not the subject of an international baby bump watch know the challenge of keeping a pregnancy private for 12 weeks or, in many cases, longer. Their reasons vary: Some fear complications or miscarriage, others simply need time to get used to the idea. And a sizable number of expectant moms worry that, despite the Pregnancy Discrimination Act of 1978, their status at work will be diminished.

The stress of keeping mum can be seen among the women taking Isis Parenting’s “Mom in the Making” class — many of them high-powered professionals — who often feel all eyes are on their bellies, said the course’s teacher, Dawn Ellis.

Angst is also clear on the message boards of TheBump pregnancy website, where users seek tips on excuses for not imbibing when out with friends. “Can you say you’ve been sick and are on antibiotics?,” one person wrote. “Say you drank too much wine the night before,” another advised.

The nervousness persists even though there has been a notable shift in the way firms operate over the years, said Betsy Myers, the founding director of the Center for Women and Business at Bentley University.

“Companies now realize that, from a profit standpoint, retaining pregnant women should be a priority,” said Myers, a former senior official in the Clinton administration. “Is what I’m seeing 100 percent [perfect]? No. But over 50 percent of new entrants into the workforce in 2010 were women. Fifty percent of middle managers in corporate America are women, and many middle managers are in their child-bearing years. If I’m a company, it doesn’t make sense from a profit standpoint [to drive them out]. If they leave, I have to replace them.”

Despite such corporate recognition, the Equal Employment Opportunity Commission reports that pregnancy discrimination claims rose from 4,287 in Fiscal Year 2001 to 5,797 in FY 2011. (A recent high came in FY 2008 with 6,285 complaints.)

“People are surprised this is still a problem,” said Christine Saah Nazer, the federal agency’s acting director of communications. “Pregnant women are having to choose between motherhood and their livelihood.”

In FY 2011, the most recent year for which statistics are available, 119 of those complaints were filed by Massachusetts workers, according to the EEOC.

Jeffrey Sankey, a Braintree-based discrimination lawyer, says the “vast majority” of employers behave the way they’re supposed to. When discrimination occurs, he noted, it’s usually subtle.

“Someone goes out on maternity leave and when they come back there’s been a re-org and they don’t have a job,” Sankey said. “The company hides behind the re-org. They don’t say it’s because of the pregnancy.

“And you certainly see it on the hiring end,” he said. “If someone is applying for a job but showing, the chances of them getting the job are significantly decreased. But of course the employer won’t say that’s the reason.”

In an attempt to strengthen existing anti-discrimination laws, in September, Senator Jeanne Shaheen (D-N.H.) and Senator Bob Casey (D-Pa.), introduced the Pregnant Workers Fairness Act (the bill is before the Senate’s Committee on Health, Education, Labor, and Pensions).

Though more than 30 years have passed since the federal Pregnancy Discrimination Act of 1978 — and in this state, protections offered by the Massachusetts Maternity Leave Act and Chapter 151B, the general anti-discrimination statute — women still worry about their jobs.