Physiology, Menstrual Cycle

Excerpt

The female reproductive system, unlike the male, undergoes regular cyclic changes known as the menstrual cycle, which serves as the body’s periodic preparation for ovulation and potential pregnancy. The most noticeable aspect of the female reproductive system is menstruation, or cyclic vaginal bleeding, which occurs alongside a series of coordinated hormonal shifts. Menstruation, also known as menarche when it first begins, typically starts around puberty with a median age of 12.4. Menstrual cycles cease at menopause, which has an average onset around age 51. Please see StatPearls' companion resource, "Physiology, Female Reproduction," for more information.

Definitions

When discussing timing within the menstrual cycle, the first day of heavy menstrual flow is considered day 1. According to the International Federation of Gynecology and Obstetrics (FIGO), normal menstrual cycles should have consistent frequency, regularity, duration, and volume of flow. Normal menstrual frequency is defined as cycles occurring every 24 to 38 days. Infrequent menstruation is defined as cycle lengths longer than 38 days, while frequent menstruation refers to cycle lengths shorter than 24 days. Amenorrhea describes the complete absence of menstrual bleeding. Normal menstrual duration is defined as bleeding lasting 8 days or less, while bleeding beyond 8 days is considered prolonged menses.

The volume of menstrual flow is classified as light, normal, or heavy. No defined objective thresholds separate these classifications, as they are often impractical in clinical settings. For research purposes, heavy menstrual bleeding is defined as blood loss exceeding 80 mL per cycle, based on weighed menstrual products. Heavy menstrual bleeding is a subjective symptom rather than a formal diagnosis. The National Institute for Health and Care Excellence (NICE) defines it as excessive menstrual bleeding that interferes with a person's physical, social, emotional, and/or material quality of life. Notably, 2 patients with the same objective volume of blood loss may have significantly different perceptions of their flow volume.

Light menstrual bleeding is rarely associated with underlying pathology, although it can occur in patients with intrauterine adhesions or cervical stenosis. For research purposes, light menstrual bleeding is typically defined as less than 5 mL of blood loss per cycle. Several factors can influence the volume of blood loss during menstruation, including medications, endometrial thickness, and bleeding or clotting disorders.

Menstrual regularity is defined by the variation in cycle lengths from one cycle to the next. Although slight variations in cycle lengths are normal, cycles are considered regular if the difference between the shortest and longest cycle lengths is 7 days or less for individuals aged 26 to 41 and 9 days or less for those aged 18 to 25 or 42 to 45. FIGO notes that for practical purposes, normal variation in cycle length can also be expressed as an average cycle length of ±4 days.

The menstrual cycle is considered irregular when cycle lengths vary by 8 days or more for individuals aged between 26 and 41 or by 10 days or more for those aged between 18 and 25 or between 42 and 45. For example, a patient aged 43 with cycle lengths of 25, 28, and 34 days has a 9-day difference between her shortest and longest cycles, indicating regular cycles for her age. In contrast, the same cycle history in a patient aged 26 would suggest an irregular cycle. Intermenstrual bleeding is defined as bleeding that occurs between cyclically regular menstrual periods. This type of bleeding can be random, meaning it is unpredictable or cyclic, indicating that it occurs consistently at the same point in each cycle.

The Ovarian and Endometrial Cycles

The menstrual cycle comprises 2 distinct cycles—one within the ovary and another within the endometrium. The phases of the ovarian cycle include the follicular phase, ovulation, and the luteal phase. The endometrial cycle consists of the proliferative phase, the secretory phase, and the menstrual phase. Generally, the ovarian follicular phase corresponds to the menstrual and proliferative phases of the endometrium, while the luteal phase of the ovarian cycle corresponds to the secretory phase of the endometrial cycle. These phases are discussed in more detail below.