Why isn't she wearing any clothes?

We attended our first "birth awareness" class today. Very informative, very fun class. The time flew by! But I have to ask, why are women in those birth videos always completely naked? Is it that they figure the whole world will see them at their most vulnerable, so why not just go all the way, or would these women naturally give birth without so much as a sports bra or tank top on? Just a question. Regardless, I'm glad there are women in the world willing to be filmed during birth because there is nothing more amazing then to see that little baby emerging for the first time. I can't believe it's only 6 more weeks until I experience it first hand!