These colorful Ying Yang balls are both 6mm and this flexible maternity belly button ring is available in 6 color combinations as shown.  The balls are attached to a flexible and comfortable PTFE bar that is 14g. As a bonus, you get a standard curved steel bar that the balls will also fit on. (shown in 2nd photo)

The PTFE bar is available in 4 maternity lengths. These bars can easily be shortened, if needed for a pefect fit.  See Rethreading Belly Charms for information.