Stress is the body's response to something that it feels is threatening or harmful. Acute stress, caused by a single event (or the fear of it), makes the heart beat faster and blood pressure go up. Breathing becomes harder, hands get sweaty, and skin feels cool and clammy. Chronic stress, which is when one is always stressed, can cause depression and changes in sleep habits. It can also decrease chances of fighting off common illnesses.
Even though infertility is very stressful, there isn't any proof that stress causes infertility. In an occasional woman, having too much stress can change her hormone levels and therefore cause the time when she releases an egg to become delayed or not take place at all.