Menstrual Cycle Regulation and Ovulation

The menstrual cycle is a recurring sequence in which the ovary and the uterine lining change in step under the control of the hypothalamic-pituitary-gonadal axis. A follicular phase of follicle growth and rising estrogen leads to ovulation, followed by a luteal phase dominated by progesterone from the corpus luteum.

Definition

The menstrual cycle is the coordinated, repeating series of hormonal and structural changes in the ovary and endometrium, organised into a follicular phase, ovulation triggered by a mid-cycle LH surge, and a luteal phase, governed by feedback among GnRH, the gonadotropins and ovarian steroids.

Scope

This topic covers the hormonal events of the ovarian and uterine cycles, the feedback switch that produces the LH surge and triggers ovulation, and the role of the corpus luteum. It treats the cycle as an integrated physiological system and is a reference account rather than clinical guidance.

Key concepts

• Follicular phase
• Dominant follicle and rising estradiol
• Mid-cycle LH surge
• Estrogen positive feedback at the surge
• Ovulation
• Luteal phase and progesterone
• Luteal regression and onset of the next cycle

Mechanisms

During the follicular phase FSH and LH support growth of ovarian follicles, and the dominant follicle secretes increasing estradiol through the two-cell, two-gonadotrophin mechanism. For most of the cycle ovarian steroids exert negative feedback, but when estradiol reaches a sustained high level it switches to positive feedback at the hypothalamus and pituitary, producing the mid-cycle surge of LH that triggers ovulation. The ruptured follicle becomes the corpus luteum, which secretes progesterone and estrogen during the luteal phase; these restore negative feedback. If the corpus luteum is not rescued it regresses, steroid levels fall, the endometrium is shed, and feedback inhibition is relieved so a new cycle can begin. Knobil's primate experiments showed that this whole sequence proceeds on a permissive, essentially unchanging pulsatile GnRH signal, placing the cyclic control largely at the level of ovarian steroid feedback.

Clinical relevance

Understanding the cycle clarifies how the timing of ovulation and the phases of the endometrium arise from shifting hormonal feedback. The estrogen-driven positive-feedback LH surge is the pivotal event that distinguishes the female axis. This entry describes normal physiology and the basis of evidence; it is not a basis for individual diagnostic or treatment decisions.

History

The cyclic nature of female reproduction was described by classical endocrinology, but the demonstration that an unvarying pulsatile GnRH signal is permissive for the cycle, with the rhythmic control residing in ovarian steroid feedback, came from Knobil's experiments in the rhesus monkey. Together with the characterisation of follicular estrogen synthesis, this established the modern feedback account of the menstrual cycle.

Key figures

• Ernst Knobil
• Stephen Hillier
• Mariastella Mihm

Seminal works

• knobil-1980
• mihm-2011
• hillier-1994

Frequently asked questions

What triggers ovulation?

A sustained high level of estradiol from the dominant follicle switches feedback from negative to positive, producing a mid-cycle surge of LH that triggers ovulation.

What happens in the luteal phase?

After ovulation the follicle becomes the corpus luteum, which secretes progesterone and estrogen; if no pregnancy rescues it, the corpus luteum regresses, hormone levels fall, and the next cycle begins.