When she asked her manager if she could leave early to go to a doctor’s appointment, Diana Antholis expected to get an unequivocal yes. After all, her co-workers were always coming in late or taking off ahead of schedule because their kids needed to get vaccinated, didn’t feel well at day care, were in a performance at school and so on.

So when the boss gave her a hard time and suggested that her medical appointments be scheduled for her “own time,” the media-agency associate almost lost it. First she thought of asking when her “own time” was, given that she typically put in more than 12 hours a day. And, second, she wanted to know why the mothers she worked with were getting time off for the asking.

“It’s as if my life, my needs, and the things I want to do are less important because I don’t have a child,” says Antholis (who’s since gone on to another job). “It’s not fair.”

It may not be fair — but that doesn’t mean it’s not widespread. As the workplace has become increasingly family-friendly over the years, there’s been an unintended consequence: complaints from childless workers — typically women — that they’re leaned on to pick up the slack for those whose attention is divided between work and family.

In an informal poll of childless working women by @work, the majority echoed Antholis’ sentiments, and formal surveys have produced similar results. In a study by the Center for Talent Innovation, 61 percent of childless women between the ages 33-47 felt their colleagues with children were given more flexibility. A 2007 study in the Journal of Vocational Behavior shows that childless workers routinely feel that they have to work harder and yet receive fewer benefits than their colleagues with children.

“Workers who don’t have children are taking on hours and duties that their parental peers are not expected to take on,” says Laura Scott, founder of the Childless by Choice Project, who has done extensive research on the matter, and says many non-parents feel exploited in the workplace. “The assumption is that they don’t have a life outside of work.”

That sounds all too familiar to Kristen Bossert, a graphic designer who’s sick of feeling like a second-class employee.

“I’m the one who always gets stuck at work,” she says. “If you have no kids, you have no excuses.”

And the griping isn’t limited to salaried, white-collar workers. One waitress, who asked that her name not be used, complained that she gets written up if she comes in to work late, while her coworkers with kids “play the baby card” and get off scot free. A shift worker at a cleaning company, who also spoke anonymously, says that “breeders” get “a free pass” where she works.

“Being a breeder is a choice, a hobby” she says. “Shouldn’t my hobbies and my choices get me [a free pass] too?”

Some complain that their lives outside of work seem invisible to both their coworkers and their employers. Their completion of a triathlon, participation in a community chorus or volunteerism don’t merit the same informal celebrations as little Johnny’s first Little League hit. In the Center for Talent Innovation study, 46 percent of women said their outside commitments are perceived as less important at work.

Employers don’t understand what a problem this is, says author and researcher Sylvia Ann Hewlett, president of the Center for Talent Innovation, a think tank focused on business issues.

“Not only are [working women without children] denied easy access to flexibility, but they’re made to feel like their lives aren’t significant and they get very little recognition of their non-work activities,” she says.

For the most part, childless women’s ire is directed less at their counterparts with offspring than at management they perceive to be unfair. In fact, Hewlett credits women without children for being generous when it comes to their coworkers and their kids.

But this generosity can have its limits, say those who have to deal with the spillover caused by their co-workers who cut their days short. Take, for example, a situation where a group is working together on a tightly deadlined project with all going well, when a concern with a child enters the picture.

“The mother gets to leave at the drop of a hat and I get the lion’s share of the work because I’m the one who doesn’t have children,” says Bossert.

Some workers without children complain that companies don’t cater specifically to their needs.

“Companies talk about being family friendly, but there’s nothing comparable for the childless,” says human resources consultant Sharlyn Lauby, who runs the popular blog HR Bartender.

Scott argues that benefits programs are loaded with perks used solely by parents, such as corporate child care centers, parental and spousal leave, company subsidized family insurance, lamaze classes and in-house lactation rooms.

While some might be tempted to tell the gripers to pipe down or grow up, there’s some evidence to support their claims of working longer hours. Women without children put in more time at work than any other group, according to Joan Williams, director of the Center for WorkLife Law. at University of California Hastings College of the Law.

But she and others bristle at the notion that parents enjoy favored status in the workplace. For one thing, the flexibility that’s built around the needs of parents does not come without a price, notes Williams, who argues that working mothers experience more discrimination than any other group in the workplace.

Women who leverage company programs such as job-sharing, which were specifically designed to keep mothers from leaving the workforce, not only take cuts in pay, but their career growth also tends to flatline, Williams and others point out.

After all, a generation of working mothers have complained about the perception that they can’t give the commitment or attention to their work that their childfree counterparts can. Having hit the “maternal wall,” they often get paid less and their opportunities for advancement become limited.

Carol Evans, CEO of Working Mother Media, points out that the corporate flexibility programs available to working mothers are typically available to all. And she notes that childless women who feel that they get the short end of the stick now may very well take advantage of flexibility benefits later, when they have a sick parent.

“For one person it might be caring for children, for another it might be caring for a parent and for still another it might be going back to school,” says Evans.

Ellen Galinsky, president of the Families and Work Institute, says the best companies are increasingly finding that workers in all stages of life are looking for more leeway at work, and saying they don’t want to be judged as to whether their need is more or less important than someone else’s.

“Whether you’re training for a marathon or taking care of a child; taking yourself to the doctor or taking someone else, your needs are both important,” she says, “and decisions around them need to be equitable.”

Brad Harrington, the executive director of the Boston College Center for Work & Family, says a growing number of companies are offering schedule flexibility on a “needs blind basis” — as with “results only” work environments where the company doesn’t want to know whether you’re going to the beach for the week, planting seeds in a community garden, or taking care of a child, as long as you get your work done.

It’s not only fairness for fairness sake that top employers are looking at, they’re also beginning to recognize that in order to attract a growing population of child-free workers, they’re going to have to provide benefits programs suited to their needs. And childless females may very well be the drivers behind further changes, especially, says Hewlett, when you consider 41 percent of professional women over 40 have no kids.

“A lot of difficult decision-making went into creating all women’s lives,” she says, “and they all want rich, full lives, whatever that may mean to them.”