If you saw my column titled "Amazing We Can Dress Ourselves" then you know that I take major exception to the idea that women have to change how they act in order to be effective in business. But, I have to protest when I hear or read about women who felt disadvantaged at work because their manager asked them to stay late and finish something that a parent couldn't finish (due to kid-related obligations). The "parents made my job harder" line is the biggest red herring ever.
"Someone had to leave to get their kid and I was forced to stay and do the work" doesn't cut it with me. Who held a gun to your head? If you have to leave, you leave. Blaming your long hours or unfair workload on a co-worker parent is the lamest excuse. The boss says, "Say, can you stay and do this project, since Amy had to get her kid?" and you say, "Gee, sorry, no."
So you don't have the 'get my kid' excuse, does that make you an automatic doormat? Particularly if this happens to you often, I'm going to suggest that you need to get some backbone. You say, "Tonight doesn't work for me" and sail out the door.
Look. In your life, your bosses will ask you to do a lot of things, some reasonable and some not. If you can do the things you're asked to do, you will. If you can't, you won't. I believe that having kids or not having kids shouldn't cause you to get special privileges or special grief at work. Your relationship with your job should be based on how successful you are at getting the work done.
There should be no magic words, like "I need to get my kids" that should cause anyone to have special privileges. But then again, the very notion of "special privileges" is an odd notion in a work environment, because every one of us is involved in dozens of micro-negotiations with our bosses every day. People negotiate better titles, more convenient hours, project assignments, and a zillion other things with their bosses every day - based on their performance - and that's how it should be.
So there is no overarching notion of FAIRNESS that can or should apply to everyone who works together in a department or a company. And that means that different schedules, different arrangements for working at home versus in the office, and other 'special deals' are perfectly legit - the boss has to make the best business decisions she can, and that often means accomodating people (based on their work results) who have special requirements of all kinds.
Your relationship with your boss is a Hub-and-Spoke one: each person is individually connected to his or her manager. The manager has the same number of Deals going as the number of people reporting to him (or her). You make your own deal with the boss, and you re-make it every day. If your deal says that you're willing to be the person who sticks around to finish projects that 'gotta-go' parents can't do, fine. That's your choice. But you can't blame those parents - or even the boss - when you make that deal. You are capable of saying that you have to go home, too, aren't you? What's stopping you?
If you fear that saying "No, I have pressing commitments this evening, I have to leave right now" will damage your relationship with your boss, you need to think about your deal some more. Maybe your deal is "my big value to the department is that I always stay and do this scutwork when other people have to bail." Okay, that's the deal you made. You don't get to complain about it! But you can change it.
I've heard people become exercised because moms they know at work get promoted even though they have this habit of rushing out to get their kids. Yes, hello, that may be the point! When you have a person - like a working mom - who isn't at work every waking hour, the boss has to really look at that mom's value to the company, and may conclude that she's indispensable. As much as companies adore face-time, lots of bosses know, in their guts, who their key value creators are. And these most valuable employees may be people who work a lot fewer face-time hours than other people. I think that's great! This is what gets me excited - companies judging people by their results.
Having looked at this issue for a couple of decades now (before and after my own kids arrived) I'm absolutely convinced that this moms-against-non-moms stuff is a function of non-parents not being comfortable saying NO.
Drew Carey gets a great laugh in his standup routine when he jokes about his geeky glasses, and then he says "And that one joke isn't enough compensation for having to wear these glasses." And look - getting to leave work early occasionally is not nearly enough compensation for having to deal with what working mothers deal with. The dark looks from co-workers alone can take years from your lifespan. And then you get home and it's utter chaos, three out of five evenings. I'm not saying "Lay off working moms." I'm saying "Take responsibility for your own personal spoke (connection to your boss), and say 'Yes' when you want to stay late, and 'No' every other time." Your colleagues who are moms are busy minding their own spokes.

