I personally credit prenatal yoga to my two wonderful natural un-medicated births.  The breath alone worked wonders, and I found the pelvis opening postures really helped to make my labor easier.  Try this simple exercise to open up your pelvis to prepare you for your labor.

Squat Circles

With your hands on your hips, step your feet about 3-4 feet apart, toes slightly turned out.  As you pilee squat circle your upper body to the right and come back to stand.  Repeat 6-8 times then repeat going to the left.

You can find this move on my 3rd trimester prenatal yoga DVD: http://fitbysara.com/prenatal-yoga.php