Menstrual Cycle

The menstrual cycle is the recurring sequence of hormonal, ovarian, and uterine changes that prepares the reproductive system for a possible pregnancy. A cycle is counted from the first day of menstrual bleeding to the first day of the next menstrual period. Its timing varies among individuals and between cycles, so a 28-day cycle is a useful reference point rather than a universal pattern .^1,2

The ovarian cycle is commonly divided into the follicular phase, ovulation, and the luteal phase. During the follicular phase, follicle-stimulating hormone (FSH) supports the development of ovarian follicles, which produce increasing amounts of estradiol. Sustained high estradiol concentrations near the middle of the cycle produce positive feedback that promotes a surge in luteinizing hormone (LH), which triggers ovulation and release of an oocyte from the dominant follicle .¹

After ovulation, the remaining follicular cells form the corpus luteum, which produces progesterone as well as estradiol. Progesterone transforms the previously estrogen-stimulated endometrium into a secretory lining that is prepared for possible implantation. If pregnancy does not occur, the corpus luteum regresses, progesterone and estradiol concentrations fall, and the functional layer of the endometrium is shed as menstrual bleeding. This marks the beginning of the next cycle .^1,2

Menstruation and the menstrual cycle are therefore not synonymous. Menstruation is the bleeding phase associated with shedding of the endometrium, whereas the menstrual cycle includes the full sequence of ovarian and uterine changes before, during, and after menstruation .¹