From Nature:
A combination of stress, dieting and exercise can dramatically affect female fertility, research on monkeys suggests. Although stress is known to reduce fertility, researchers now warn that if a woman is also dieting and exercising, the effect could be many times greater. In stressed women, increased levels of a hormone called cortisol block the signal from the brain that tells the ovaries to release eggs, explain Sarah Berga and her colleagues at Emory University School of Medicine in Atlanta, Georgia.
In severe cases, a woman can stop producing eggs altogether and have her periods cease — a condition called amenorrhoea. About 5-10% of women suffer from amenorrhoea and Berga has previously found that giving such women behavioural therapy to control stress levels can help restore periods and fertility, without the need for specific fertility treatment.
More here.