It's called a BeBand and it's magical!
It's a band of fabric you wear over your pants and underneath a shirt to cover up jeans or skirts that you can no longer button. I will tell you that it is definitely worth the extra $10-15 it costs in stores. I can wear jeans with normal length shirts again! I have completely worn out all of my longer shirts that could hide my unbuttoned/unzipped jeans, so it's nice to be able to wear my other shirts again. The BeBand makes it look like you layered your shirt with a tank top, which is "in" regardless. So I give props to whoever invented this handy little maternity band!
Below are two pictures of me modeling the BeBand at 30 weeks pregnant.
Here are two maternity band links by Topshop that cost $14.00.
If you don't own one of these maternity bands yet, check out my first solution to Wearing Jeans While Pregnant.