Wait... what does that mean?
So, there are a few different ways we can breathe. We can breathe UP. Watch someone breathe this way or try it yourself- your collar bones move up and down when breathing. This way isn't good because it causes issues with the cervical spine and neck. The other way we can breathe is DOWN. Into the belly. Plunging everything downward with every breath. This isn't good either because it increases intra-abdominal pressure and pushes organs downward. I definitely think those of us who tend towards belly breathing are the ones who feel this "I can't get enough breath!" when breathing during pregnancy because baby is restricting the ability to do what we want to do- breathe DOWN. The third way we can breathe is OUT- into the ribs. Our ribs are designed to MOVE torsionally. That is, they kind of flare outwards during breathing (Don't worry- I'll post a video). This helps with bone density of the ribs. And it doesn't cause any other issues during breathing- there are no compensations that our body has to make to breathe into the rib cage.
Here are a few stretches I like: