Yoga practice emphasizes breathing through the nose. If you are breathing through your mouth you are likely to be over-breathing. It is necessary to create an awareness of breathing patterns when approaching yoga. The benefit of placing the body in various postures is to expand the breath - by introducing particular energetic and muscular locks in the body.
Sometimes, you use the intercostal muscles (muscles between the ribs) more when breathing, expanding the ribs. At other times, you may be using your abdomen more, keeping the chest somewhat still.
You may find that you can remove all force and control of the breath and let the body's position dictate how you breathe. The breath is not forced, in that case, but is a natural result of the position of the body and the type of movement.
As you slow down the breath, there is a natural pause between breaths. Without force or strain, you become a more efficient breather and a fitter, healthier and happier person.