Ovarian Cycle

The ovarian cycle consists of two main phases: the follicular phase and the luteal phase. The follicular phase, averaging around 14 days, involves the growth and maturation of ovarian follicles stimulated by follicle-stimulating hormone (FSH). The luteal phase, lasting approximately 14 days, follows ovulation and is characterized by corpus luteum activity. Together, these phases typically complete a 28-day cycle, though cycles can range from 21 to 40 days.

A primordial follicle transforms into a primary follicle when the primary oocyte secretes the zona pellucida, a glycoprotein-rich extracellular layer. The primary follicle then develops multiple layers of granulosa cells and becomes a secondary follicle. As development continues, theca folliculi—connective tissue and epithelial cells—form around the secondary follicle. Finally, the secondary follicle matures into a vesicular or antral follicle containing a fluid-filled cavity called an antrum.

Ovulation is triggered by a surge in luteinizing hormone (LH), which causes the mature follicle to rupture. During ovulation, the secondary oocyte, surrounded by the corona radiata (a capsule of granulosa cells), is released into the peritoneal cavity. Some individuals experience mittelschmerz, or ovulatory pain, during this process, though the precise mechanism remains unclear. In rare cases (1-2%), two oocytes may be released, potentially resulting in fraternal twins.

The corpus luteum forms from the ruptured follicle after ovulation and secretes large amounts of progesterone along with smaller quantities of estrogens, relaxin, and inhibin. Progesterone prepares the endometrium for implantation, while inhibin reduces further FSH secretion. If pregnancy occurs, human chorionic gonadotropin (hCG) from the developing embryo sustains the corpus luteum until the placenta can produce adequate hormones independently.

During the follicular phase, FSH promotes the growth of multiple follicles while preventing atresia. Among these follicles, a dominant follicle emerges and becomes increasingly sensitive to FSH by up-regulating its FSH receptors. This dominant follicle is also sensitive to LH, allowing it to mature fully at ovulation while its counterparts regress. The primary oocyte within the dominant follicle undergoes meiosis I, forming a secondary oocyte and polar body in preparation for ovulation.

Without pregnancy, the corpus luteum degenerates into the corpus albicans, a scar-like remnant. As the corpus luteum degenerates, progesterone levels decline, triggering endometrial shedding and the onset of menstruation. This hormonal shift marks the transition from the luteal phase back to the follicular phase, restarting the ovarian cycle. The corpus albicans remains as a permanent scar on the ovary.

While a 28-day cycle is common, ovarian cycles range from 21 to 40 days, with variability largely due to differences in the follicular phase and ovulation timing. The follicular phase is variable and can be influenced by hormonal factors and individual physiology, whereas the luteal phase remains relatively consistent at approximately 14 days. Additionally, some individuals experience variations in oocyte release influenced by factors beyond hormonal signaling, potentially explaining rhythm method contraceptive failures.