Liquidsky: I think this is not so much a medical problem as a social problem. A woman in labour may be horrified to poop over the midwife, child and obstetrician, even if this has no medical adverse consequences, or may be anxious not to do so.
Plus, at least my wife tells me from personal experience, when one is pregnant, urges to poop or pee have a particular urgency (not enough room).
So I guess our maternity ward advised the glycerin suppository and pooping before going to the hospital rather for the mother's comfort and ease of mind rather than medical reasons.
(A comparison comes to mind: apparently, some people are horrified that physicians may see them with dirty underwear. This is of course silly, since physicians, especially those dealing with emergencies, see much worse things, but...)
(Another comparison comes to mind: my mother was horrified at the idea that somebody, especially a male, could see her peeing - as if she was the center of the attention of the universe. So if we were on a hike or road trip and she felt a strong urge to pee, she would resist all suggestions to go behind a bush, and would be obnoxious until reaching a real toilet. This is irrational.)