Here is a really interesting article on a new study that was recently published. And this is the abstract for the original research publication.
If you are in the situation of not geting your period, exercising a lot, and perhaps undereating, it is well worth considering the paragraph at the end of the article "To seal the conclusion that a negative energy balance was the key to exercise-induced amenorrhea, the researchers took four of the previous eight monkeys and, while keeping them on the same exercise program, provided them with more food than they were used to. All the monkeys eventually resumed normal menstrual cycles. However, those monkeys who increased their food consumption most rapidly and consumed the most additional food, resumed ovulation within as little as 12 to 16 days while those who increased their caloric intake more slowly, took almost two months to resume ovulation."
This strongly suggests that if you are interested in resuming ovulation naturally, you eat more (and perhaps exercise less). The more quickly you can adjust your energy balance from being negative to positive, the more quickly you will regain your cycles.
If you want to be pregnant, look at it this way - do you want to be thin, or pregnant? It *is* a choice! And the thing is, even if you do increase your intake and decrease your exercise, you won't blow up. The experience of many women in this boat with whom I have been in contact is that while you may gain quite a bit of weight at the beginning, it's the weight that your body needs for fertility. You will find a setpoint, probably at a BMI of 23 or lower, at which your gain will slow and then stop. And honestly, even at a BMI of 23, you are very very far from being overweight. It's actually healthy. And your body will very likely respond to that health by cycling again.
Among the women with whom I have corresponded, those who have put on 10, 15 lbs in the shortest amount of time have been those that have started cycling again most rapidly, just like the monkeys described in the paragraph above. Food for thought!!!