This doesn't seem physically possible... guess that's one of the nice things about drawings, they're not bound to silly things like anatomy. I do have to ask how she got it in there... shoe horn anyone?
You can tell which women have given birth in their bones by the fact that childbirth will scrape or even break the pelvis. It is possible to shatter the pelvis and force something rather large through the resulting gap (like a baby's skull).
...She wouldn't be able to stand while this was happening, though... Maybe it was previously shattered, and the bones set in a wider-than-usual manner? Or maybe she just has a very large gap in her pelvis/wide hips by accident of birth...
As a total side note, that girl is pretty cute, I like that kind of glasses/meganekko. Reminds me of that girl from Melody of Oblivion.