I only wear skirts and dresses also. And that particular skirt has a lot of yardage, with 3 different levels of tightness to adjust the waist, one size fits all (well, hemmed mine because I'm 4'10"). I tie mine in the back with a square knot and then put the straps down the inside of the skirt because I don't care for the bump the bow makes when my shirt isn't tucked. Looks to me like they will last all 9 months - I hope.
And they do carry a maternity dress too, in denim, black, and biege:
http://www.tznius.com/cgi-bin/group.pl?id=41