if your mind is worry free and expecting a smooth, easy birth, your body will be more likely to produce endorphins, natural relaxation and pain-blocking hormones. In this relaxed state of body and mind, the uterine muscles work efficiently and smoothly allowing the natural birthing process to proceed calmly and smoothly. You may still experience tightening or pressure, but not severe level of pain that many women have come to expect.
On the other hand, if your mind is full of fear and negativity, then your body will be more likely to produce stress hormones which constrict your uterine muscles, creating pain. The fear-tension-pain syndrome sets in. Your muscles may tighten up, you may experience increased pain and tension, and the natural birthing process may stall. This is unfortunate, because all you may really need is to relax and trust your body.