The Ovarian and Uterine Cycles and Their Hormonal Conductor

The Story

The menstrual cycleThe monthly cycle of changes in the female reproductive system. is two synchronized cycles running at once. In the ovary, the follicular phaseThe first phase of the ovarian cycle when follicles grow and estrogen levels rise. grows a follicle under follicle-stimulating hormone (FSH); rising estrogen from that follicle triggers a luteinizing hormone(LH): Triggers ovulation and testosterone production. (LH) surge that causes ovulationThe release of a mature oocyte from the ovary.; the emptied follicle becomes the corpus luteum, which secretes progesteroneA hormone that supports pregnancy and regulates the menstrual cycle. in the luteal phaseThe second phase of the ovarian cycle, dominated by progesterone secretion.. In the uterusThe muscular organ where a fertilized egg implants and develops., the endometriumThe inner lining of the uterus that thickens during the menstrual cycle to support a potential pregn runs in parallel: it proliferates under estrogen (proliferative phase The phase when the endometrium rebuilds after menstruation.), becomes secretory and receptive under progesterone (secretory phaseThe phase of the menstrual cycle when the endometrium is prepared for implantation.), and — if no pregnancy occurs — sheds as menstruation when the corpus luteum dies and progesterone falls. The two cycles are locked together because the same ovarian hormones drive both.

Conducting all of it is the hypothalamic-pituitary-gonadal (HPG) axisSecond cervical vertebra; has the odontoid process (dens) for pivoting head (“no” motion).: the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions. releases GnRH in pulses, the anteriorThe front of the body or toward the front when standing in the anatomical position. pituitary answers with FSH and LH, the ovary responds with estrogen and progesterone, and those gonadal hormones feed back on the hypothalamus and pituitary — usually negative feedbackA control mechanism that reverses a change in the body to maintain homeostasis., but with a critical mid-cycle switch to positive feedbackA control mechanism that amplifies a change instead of reversing it. that fires the LH surge. This is the same feedback-axis logic Stina’s endocrine module (M12) taught with cortisolA glucocorticoid involved in stress response, metabolism, and immune regulation. and PTH, now controlling reproductionThe process of producing offspring.. Hold onto one idea: the precision of this loop is its vulnerability. A system that depends on clean, well-timed hormonal pulses can be thrown into heavy, irregular bleeding by anything that adds noise to the signal.

From Stina’s chart: Stina’s bleeding was not only heavy but irregular in timing — a hint that the hormonal rhythm of the cycle itself, not just the lining, was disturbed.