Now through March 31 at Thyme Maternity, you'll get $50 in "Thyme Fashion Cash" when you spend $100 or more. The $50 card can be redeemed from April 29 to May 14 on purchases over $100. This offer is valid in-store only, but if you're shopping online, they are offering 20% off everything (including sale items) when you shop through their online boutique at Toys R Us. (You'll also get free shipping over $100.)
If you have a big quantity of maternity clothes to buy, the $50 card with a $100+ purchase gives you a bit more bang for your buck with a total of 25% off. Although stock/sizing is limited, it's probably worth checking out the online clearance section to see if there's anything that works for you.