In some styles you can simply size up (and some maternity brands sell non-maternity pieces in different sizing and pass them off as maternity clothes). This sizing up tactic works best for pieces that aren't super fitted around the bump. For example, you can simply size up in a smocked dress that is fitted in the bust and then loose and flowy. When it comes to pieces that are more fitted around the bump, stick with maternity specific styles. Most of these have added fabric or ruching in the stomach to accommodate a growing bump. Plus, maternity clothes are also purposefully loose or comfortable in the waist and you'll likely find they just feel better on.