Interesting question. There is a good article on Associated Content that asks this question.
My thinking is that if moms are listening to affirmations throughout their pregnancy then they are more empowered and stand up for their rights and trust their bodies. All of these things help a mom make more informed choices and hoepfully have better births!
There is a great comment about the Birthing My Twins Naturally CD on the Hypnobabies Review Blog, where a mom felt that her affirmations really helped her stand strong in a discussion with her care provider.
Here is a blog post from a mom who uses affirmation in her pregnancy and how it helps her. It includes her long list of affirmations! Good for her!