It's one size fits most with a Velcro closure, so no need to worry about getting the right size AND you don't have to worry about it not fitting if your size changes in the postpartum period
Here's how long the velcro closure is, so it's very flexible in terms of sizing
One of the things I really like is that it's super easy to use with Spectra flanges (with the combined flange/connector). It feels very secure.