Hypothalamic amenorrhea (HA) is one of the most common causes of missing periods, which occurs when the major control center in your brain, the hypothalamus, stops releasing the hormones needed for ovulation and menstruation. In this episode, Dr. Nicola Rinaldi, author of "No Period. Now What?," talks to us about the common triggers of HA — dieting, overexercise and stress — and how to heal.