This morning on the radio they were discussing "3 things you should never say to a mom." Their list included 1) you look tired, 2) you have your hands full and 3) (the dreaded) do you work?
The "tired" statement makes it easy for a mom to infer that she must look bad/ugly/terrible/a wreck because who ever looked "good" tired? The "hands full" statement leaves the sense that the kids are out of control, which implies the mom isn't doing a good job. The "do you work?" statement -- that one need no further explanation.
But I got to thinking that's not all of them. This blog on Yahoo brought to light a few more: "when did you get so gray?"; "when are you due?"; and for moms of boys, "you must want a girl."