What is the menstrual cycle?

The Menstrual Cycle Explained

The menstrual cycle is a coordinated hormonal process lasting 21-35 days in adult women, consisting of three distinct phases (follicular, ovulatory, and luteal) that prepare the body for potential pregnancy through the maturation and release of an egg, followed by either implantation or menstruation. 1

Cycle Duration and Timing

• Normal cycle length ranges from 21-35 days in adult women, with adolescents showing wider variability of 21-45 days as their hormonal systems mature 1
• Menstrual bleeding typically lasts 3-7 days, with cycles occurring at regular intervals within the normal range defined as eumenorrhea 1, 2
• Ovulation generally occurs between days 9-20 in a standard 28-day cycle, though this timing shifts earlier in shorter cycles and later in longer cycles 3

The Three Phases of the Menstrual Cycle

Follicular Phase (Days 1-14 in a 28-day cycle)

• This phase begins on the first day of menstruation and ends at ovulation, characterized by rising FSH levels that stimulate follicle growth in the ovaries 4
• FSH stimulates a cohort of follicles to grow, with one dominant follicle eventually selected in the mid-follicular phase to continue developing while others regress 5
• The dominant follicle produces increasing amounts of estradiol, which stimulates the endometrial lining to proliferate and thicken in preparation for potential implantation 4
• Follicular phase length is the most variable component of the cycle and tends to shorten with advancing age, while the luteal phase remains relatively stable 6, 5

Ovulatory Phase (Mid-cycle, approximately 24-36 hours)

• A critical concentration of estradiol from the dominant follicle triggers positive feedback in the hypothalamus, causing a surge in LH (luteinizing hormone) that initiates ovulation 4
• The LH surge causes the mature follicle to rupture and release the egg (oocyte) from the ovary, typically occurring 24-36 hours after the surge begins 3
• The fertile window spans approximately 5 days before ovulation through the day of ovulation, as sperm can survive up to 5 days in the female reproductive tract 3

Luteal Phase (Days 15-28 in a 28-day cycle)

• After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone and estradiol in response to LH pulses 5, 4
• The corpus luteum reaches peak function 6-7 days after ovulation, producing maximal progesterone levels that prepare the endometrium for implantation 5
• Progesterone causes the endometrium to become secretory and differentiated, creating optimal conditions for embryo implantation during the mid-luteal phase 4
• If pregnancy does not occur, the corpus luteum regresses passively, leading to declining progesterone and estradiol levels that trigger menstruation 5
• Mid-luteal progesterone levels above 6 nmol/L confirm ovulation occurred, while levels below this threshold indicate anovulation 7

Hormonal Regulation

• GnRH (gonadotropin-releasing hormone) pulses from the hypothalamus set the tempo, occurring every 1-1.5 hours in the follicular phase and every 2-4 hours in the luteal phase 4
• The pituitary gland responds to GnRH by secreting FSH and LH, which stimulate the ovaries to produce sex hormones and mature follicles 4
• Theca cells in the follicle produce androgens in response to LH, while granulosa cells convert these androgens to estradiol under FSH stimulation 4
• Two-thirds of women show two follicle waves per cycle, while one-third show three waves, with three-wave women having longer cycles and later ovulation 5

Menstruation

• Menstruation occurs when progesterone withdrawal triggers endometrial shedding in the absence of pregnancy, initiated by decidual cells and executed through prostaglandins, vasoconstriction, and matrix metalloprotease activity 5
• The functional layer of the endometrium is shed during menstruation, while the basal layer remains intact to regenerate the lining for the next cycle 5
• If pregnancy occurs, hCG from the trophoblast prevents corpus luteum regression, maintaining progesterone production to support the early pregnancy 5

Common Pitfalls and Clinical Considerations

• Adolescents may experience anovulatory cycles initially after menarche, but the vast majority of cycles become regular within 1-2 years, with persistent irregularity warranting evaluation 2
• Cycle length variability is normal, with the follicular phase being the primary source of variation rather than the luteal phase 6, 5
• Baseline hormone measurements should be taken on days 3-6 of the cycle for accurate assessment, calculated as the average of three measurements taken 20 minutes apart 7
• Hormonal contraceptives suppress normal FSH and LH patterns and should be discontinued at least 2 months before baseline reproductive hormone testing 7