Menstruation
Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days).
Functional hypothalamic amenorrhea
Females who have menstrual cycles lasting longer than 45 days and/or amenorrhea for three or more months should be evaluated for FHA. Differentiating FHA from the irregular menstrual patterns seen in adolescents during the initial years after menarche due to immaturity of the HPO axis can be challenging. However, studies have shown that even during this period, the length of a menstrual cycle does not exceed 45 days. Furthermore, healthy girls with normal BMI (18.5-25 kg/m) should develop regular menstrual cycles (every 28 +/- 3-5 days) within 1-2 years after menarche. FHA is a diagnosis of exclusion, and thus the evaluation should be used to rule out organic causes of amenorrhea (e.g., pregnancy, thyroid disorders, inflammatory bowel disease, etc.) Evaluation for FHA may include a thorough history and physical exam, laboratory testing, and imaging if appropriate. The Endocrine Society Clinical Practice Guidelines on Functional Hypothalamic Amenorrhea (FHA) suggests obtaining a baseline bone mineral density measurement by DEXA scan from any patient with 6 or more months of amenorrhea.