The menstrual cycle is a crucial indicator of a woman's health, akin to vital signs like heart rate or blood pressure. The primary event in the menstrual cycle is ovulation, which signifies a healthy functioning of the endocrine and reproductive systems. If ovulation does not occur, any bleeding experienced is not considered a true menstrual bleed.

The American Academy of Pediatrics (AAP) and The American College of Obstetricians and Gynecologists (ACOG) have both emphasized the importance of monitoring menstruation as a vital sign in adolescent girls. They suggest that any irregularities or abnormalities in the menstrual cycle can serve as early warning signs of potential health issues.

For instance, irregular ovulation could be an indication of an underlying health problem. Symptoms such as delayed ovulation, mid-cycle spotting, abnormal cervical mucus patterns, irregular periods, or amenorrhea (absence of menstruation) could be signs of an underlying health issue.

The menstrual cycle consists of two main stages: the preovulatory (follicular) phase and the postovulatory (luteal) phase. Fertile days occur in the preovulatory phase, leading up to ovulation, while the rest of the cycle is infertile once ovulation is confirmed. For effective use of fertility awareness as a birth control method, it's essential to differentiate between fertile and infertile days.