Yoga teaches to come in harmony with the breathing and therefore is a help for preparation leading to and during the whole birth event.
The in-breath is carried out by the nose, breathing out by the mouth (like candle out a candle) which is slightly opened and relaxed.
There is a reflective connection between mouth and pelvic floor, the pelvic floor muscles relax when the lips are relaxed.

The 3-Part Breath during Pregnancy.

(1) Breathing in the Belly.
This breath envolves the intestine,stomach,spleen,liver and gallblader region.
The breath (prana) surrounds the developing child, the physical movement of the breath in the belly area massages not only the organs but more importantly the baby too.
Abdominal breathing is restricted at the end of the pregnancy, the intestines have less space and this affects the emotional system.

(2) Breathing in the Chest.
This breath movement involves the lungs,ribs and diaphragm.
During pregnancy the in-breath has to work a little harder as the downward movement of the diaphragm presses against the resticted space below.
However the Out-breath becomes easier as the fuller space below has more tendency to push upwards. 

(3) Breathing in the Collarbone
This breath envolves the upper lobs of the lungs,collarbone,scapular and shoulder joints
Collarbone breathing touches the thinking, concentration, and intuition.

The goal is to breath in all 3 areas ie first belly then chest and to finish high in the collarbone area.

Important: When you feel everthing is going wrong - work-family-neighbours etc take a deep breath and "Sigh" breathing out and dropping and relaxing the shoulders , continue the breath down to the pelvic floor and it will let the steam out of the pressure cooker.