STYLETHREAD -- LET'S TALK SHOP!

So as I mentioned in another post I have a girlfriend whose pregnant and has asked me to help keep her "stylish" through her pregnancy.  I'd love to help her considering I can use the information for myself when/if I become pregnant.  She's looking to purchase a pair of designer denim maternity jeans.  She's not that far along and doesn't need them yet, but she's excited about ordering them and probably will need them in another month.  I've been checking on E-bay at the Citizens and True Religions (my favorite choices of course) and I'm confused on the sizes.  The seller will say something like - Size 30 (pre pregnancy) = Medium in Maternity.  But then it says they suggest ordering one size up from your prepregnancy size.  So do the jeans simply come in Small, Medium, and Large and the numbers are only to help you determaine what you might need.  So if she was a 30 in Citizen Jeans - she should order a medium and they'll fit for a few months??  I hate to bother you guys with this but I can't really help her, and I want to try!