The more I think about it, and the more I talk to working moms across the U.S., the more committed I become to the idea that much of our difficulty in the workplace can be attributed to wretchedly poor management. There are just an enormous number of people managing others who should not be in those positions.
They don't know how to evaluate results, so they pay attention to when you arrive, and when you leave.
They don't know how to gauge progress on deliverables, so they demand evidence of your commitment in the form of sacrificing personal obligations for the sake of the job. Now, there's a great employee!
They don't know a thing about two-way communication, so they tell you what they want and don't ask whether, from your vantage point (as the person who has to get this thing done) their way is the best way to do it.
There is something subversive about a working mom - she has something in her life that's more important to her than you are - you, the one who's paying her. (Okay, it's actually the company that is paying her, but heck - I'm still her boss. Can I get some respect over here?)
How dare she be so excited over the three-year-old's first ice-skating class? Can we spare some of that excitement for our product launch next month?
There's something annoying about a working mom. She's too fulfilled. We need people who are so committed to their jobs that nothing, NOTHING will stop them from working when the company needs them to.
That "my kids need me" excuse is sneaky. I, the manager, can't fight it, or I'd look like a kid-hating slimebag. I much prefer employees over whom I can exert more control. These working moms are a pain in the butt.

