Menstrual Cycle and Hormonal Coordination

Two linked cycles occur in different organs

The menstrual cycle is a coordinated sequence of changes involving ovarian follicles, the uterine lining and endocrine signals. It begins with the first day of menstrual bleeding and continues until the first day of the next bleeding episode. The chapter uses an illustrative average interval of around 28–29 days, but actual cycles vary between individuals and within the same individual. Ovulation does not always occur on calendar day 14. A useful description must link two processes: the ovarian cycle of follicular growth, ovulation and luteal activity, and the uterine cycle of menstruation, endometrial proliferation and secretory preparation.

Menarche is the first menstrual bleeding episode associated with the onset of reproductive maturation. Menopause describes the permanent end of menstrual cycles due to loss of ovarian follicular function, usually occurring in midlife, although its timing varies. These are biological transitions rather than abrupt simultaneous changes in all reproductive functions. During an ordinary cycle, the endocrine activity of the hypothalamus, pituitary and ovaries determines the growth and shedding of the endometrium; the uterus does not operate a separate clock independently of ovarian hormones.

Menstrual and follicular phases

The menstrual phase begins when levels of ovarian steroid hormones fall after regression of the corpus luteum, provided pregnancy has not established the hormonal signals needed to maintain it. The functional endometrial layer loses hormonal support and part of its tissue, blood and secretions exits via the vagina. In the textbook's example, menstrual flow may last about three to five days; the duration varies. Bleeding should not be described as proof that an egg itself is being expelled as the visible menstrual fluid. Menstruation reflects cyclical changes in the uterine lining.

During the ovarian follicular phase, FSH promotes growth of a group of follicles, one of which often becomes dominant. As follicles grow, their granulosa and theca cells cooperate to produce increasing amounts of estrogens, especially estradiol. Rising estradiol stimulates proliferation and repair of the endometrium after menstruation. LH also participates in ovarian steroid production. Although several follicles may initially grow, a dominant follicle normally reaches the preovulatory stage while many other follicles undergo atresia.

The mid-cycle LH surge and ovulation

When estradiol rises to a sustained high level during the late follicular phase, endocrine feedback temporarily shifts toward a positive-feedback pattern. This contributes to a rapid surge in luteinising hormone, usually accompanied by a smaller change in FSH. The LH surge triggers final maturation events in the dominant follicle, resumption of oocyte meiosis I and ovulation. The follicle ruptures and releases a secondary oocyte. In a typical 28-day cycle this is often illustrated near the middle of the cycle, but ovulation timing varies substantially and cannot be established from cycle day alone.

The LH surge is not the same as an embryo implanting or as fertilisation. Ovulation takes place whether or not sperm are present. After ovulation, the released oocyte enters the oviduct through the action of the fimbriae and tubal transport mechanisms. If fertilisation is to occur, viable sperm and oocyte must meet within a limited interval. The ovarian follicle that has released the oocyte remains in the ovary and changes into a corpus luteum, an endocrine structure rather than another developing egg.

The luteal phase and endometrial preparation

During the luteal phase, the corpus luteum secretes substantial progesterone along with estrogens. Progesterone acts on the endometrium, promoting glandular secretion and a tissue environment suitable for possible implantation. The endometrium is therefore in a secretory rather than a merely proliferative state. The corpus luteum is hormonally active even when fertilisation does not take place. If no developing embryo implants and produces the appropriate hormonal support, the corpus luteum regresses and progesterone and estrogen levels decline, which initiates the next menstrual bleeding episode.

When implantation occurs, trophoblast-associated signals such as human chorionic gonadotropin (hCG) act to preserve the corpus luteum in early pregnancy, maintaining progesterone production. This prevents the usual withdrawal of hormonal support and ordinarily stops cyclic menstruation during pregnancy. However, the absence of menstruation alone is not a reliable diagnosis of pregnancy: stress, illness, hormonal conditions and other factors can also interrupt cycles. Likewise, not all vaginal bleeding is a normal menstrual period.

Feedback between the hypothalamus, pituitary and ovaries

Gonadotropin-releasing hormone from the hypothalamus stimulates anterior-pituitary secretion of FSH and LH. In the follicular phase, ovarian estrogen and inhibin influence the pituitary and hypothalamus through feedback, and one follicle often becomes dominant. Sustained high estrogen then supports the mid-cycle LH surge. After ovulation, progesterone from the corpus luteum changes the feedback environment and maintains the endometrium. Hormonal curves are therefore consequences of a multi-organ feedback network and should not be read as independent hormones turning on one at a time.

The ovarian follicular phase can vary considerably in length, whereas the postovulatory luteal phase is typically less variable. Consequently, one cannot accurately infer ovulation by always subtracting the same number from a calendar date without acknowledging variability. Menstrual hygiene involves using clean, suitable absorbent products, changing them according to the product and actual flow, washing hands and disposing of used products safely; this supports comfort and reduces contamination. The reproductive biology of the cycle is not determined by the particular product used.

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