It's become a Mother's Day tradition on a par with candy, flowers and guilt. While advertisers wax poetically about the priceless work of motherhood, economists tally up the paycheck for the services she performs.

This year, salary.com estimates the value of a full-time mom at $138,095, up 3 percent from last year. The monetary value of a second-shift mom is $85,939, on top of her day job.

But, alas, the check is not in the mail. Nor will mom find it next to the maple syrup on her bed tray. Motherhood is what the economists call a monopsony, a job for which there is only one employer. And it's a rare child who's saved up to fill mom's piggybank, let alone a 401(k).

The real story of the Mother's Day economy is less rosy. This is what to expect when you are expecting — expecting to be a mom and a paid worker at the same time. You can expect to be mommified.

Mothers are still treated as if they were a third gender in the workplace. Among people ages 27 to 33 who have never had children, women's earnings approach 98 percent of men's. Many women will hit the glass ceiling, but many more will crash into the maternal wall.

Here's a Mother's Day card from a study just published by Shelley Correll in the American Journal of Sociology. Correll performed an experiment to see if there was a motherhood penalty in the job market. She and her colleagues at Cornell University created an ideal job applicant with a successful track record, an uninterrupted work history, a boffo resume, the whole deal.

Then they tucked a little telltale factoid into some of the resumes with a tip-off about mom-ness. It described her as an officer in a parent-teacher association. And — zap — she was mommified.

Moms were seen as less competent and committed. Moms were half as likely to be hired as childless women or men with or without kids. Moms were offered $11,000 less in starting pay than non-moms. And, just for good measure, they were also judged more harshly for tardiness.

"Just the mention of the PTA had that effect," says Correll. "Imagine the effect of a two-year absence from the work force or part-time work."

If this is true in the lab, it's true in real life. Joan C. Williams, who runs the Center for WorkLife Law at Hastings Law School, says discrimination against women may have gone underground but "the discrimination against mothers is breathtakingly open. Mothers are told, 'You belong at home with the kids, you're fired.'"

In the stories from the center's hot line and in the growing case law they've accumulated on family responsibility discrimination, you hear about women overtly denied promotions for having a child, told to have an abortion to keep a job, or rejected for a new job because "it was incompatible with being a mother." Family emergencies are treated differently than other timeouts. And things are at least as bad for dads when they take on mommy's work of caregiving.

I'm not suggesting that mothers quit the PTA, hide the kids or even sue, although the 400 percent increase in FRD suits has, um, raised some corporate consciousness. But at the very least, we have to turn the story line around.

No, mothers are not actually a third gender. More than 80 percent of American women have children and 80 percent of those are employed by the time their kids are 12. The reality of the workplace affects us all.

The much-touted mommy wars are as useful in solving our problems as a circular firing squad. And tales of women "opting out" of professional careers squeeze out the tales of women being pushed out.

As for the idea that women's lives are an endless array of choices? Williams says ruefully, "An awful lot of what gets interpreted as a mother's choice to drop out is really a 'take this job and shove it' reaction by mothers who encounter discrimination."

How many mothers would choose to spend more time at home if the fear of re-entry weren't so daunting? How many would choose to stay in the work force except for one sick child, one snow day, one emergency room visit? And how many dads would choose to live up to their own family ideals?

On Mother's Day 2007 there is still a deep-seated bias that puts the image of a "good mother" at odds with that of an "ideal worker." Until we wrestle down the beliefs and the rules of the workplace, our annual homage to the family values keeper will be as sentimental as this year's $138,095 paycheck.

Ellen Goodman is a columnist for The Boston Globe. Reach her at ellengoodman@globe.com.