We recommend staying with your pre-pregnancy size (unless indicated on the product size chart).
If you choose the larger size you may find them too big in the bottom and thighs, and will be hitching them up all the time. The jeans may feel a little snug at first but remember that the denim will give with each wear and wash.
As a rough guide, I am generally an Australian size 10 in clothing. In maternity jeans, I fit: Mavi size 26, Seraphine size XS, Soon size XS, Maternal America XS, Queen mum size S, Ripe size XS, Esprit size S, Noppies size S and Supermom size S.