Now this one is REALLY interesting. Reading the description, it sounds like it's either/or: either you can wear it and just look stylish, OR you can wear it as a maternity dress, because it's adjustable like that.
How cool is that? Sadly, it's sold, but a great example of 1930s maternity-wear.
Haven't been able to track down a 1920s maternity style yet, so if someone has one, leave a comment and I'll post it tomorrow.