Describe menstrual cycle. What is the duration of menstrual cycle in women?

The menstrual cycle is the regular, recurring series of physiological changes that occur in the female reproductive system, preparing the body for potential pregnancy each month. This cycle is orchestrated by a complex interplay of hormones produced by the hypothalamus, pituitary gland, and ovaries. The cycle involves two main coordinated processes: the ovarian cycle (changes in the ovaries) and the uterine cycle (changes in the uterine lining), both controlled by fluctuating hormone levels. Understanding this cycle is fundamental to reproductive health, family planning, and recognizing when irregularities might signal health concerns.

The menstrual cycle typically lasts 28 days on average, though normal cycles can range from 21 to 35 days in adults and 21 to 45 days in young adolescents. The cycle is counted from the first day of menstrual bleeding (day 1) until the day before the next period begins. The cycle has four main phases. First is the menstrual phase (days 1-5), when the uterine lining (endometrium) breaks down and is shed through the vagina as menstrual flow (period), consisting of blood, mucus, and tissue. This occurs because if no pregnancy happened in the previous cycle, progesterone levels drop, triggering the shedding. Second is the follicular phase (days 1-13), which actually overlaps with menstruation. During this phase, follicle-stimulating hormone (FSH) from the pituitary gland stimulates several follicles in the ovaries to grow, each containing an immature egg. Usually, one follicle becomes dominant. As follicles develop, they produce increasing amounts of estrogen, which causes the uterine lining to thicken and regenerate, preparing for potential pregnancy.

The third phase is ovulation (around day 14 in a 28-day cycle), the pivotal event when the mature egg is released from the dominant follicle in the ovary. This is triggered by a surge in luteinizing hormone (LH) from the pituitary gland, responding to peak estrogen levels. The released egg travels into the fallopian tube where it can be fertilized by sperm if sexual intercourse has occurred. The egg remains viable for about 12-24 hours, though sperm can survive in the female reproductive tract for up to 5 days, creating a "fertile window" of approximately 6 days. The fourth phase is the luteal phase (days 15-28), during which the empty follicle transforms into a structure called the corpus luteum, which produces large amounts of progesterone and some estrogen. Progesterone maintains and further prepares the thickened uterine lining for embryo implantation. If fertilization and implantation occur, the developing embryo produces human chorionic gonadotropin (hCG), which maintains the corpus luteum and progesterone production, preventing menstruation and supporting early pregnancy. If fertilization doesn't occur, the corpus luteum degenerates after about 14 days, causing progesterone and estrogen levels to drop sharply. This hormonal decline triggers the breakdown of the uterine lining, beginning menstruation and starting the cycle anew.

Individual variation in cycle length is normal, and cycles often irregular during the first few years after menarche and approaching menopause. Factors affecting cycle regularity include stress, significant weight changes, excessive exercise, illness, hormonal imbalances, polycystic ovary syndrome (PCOS), thyroid disorders, and certain medications. Tracking menstrual cycles helps women understand their body's patterns, identify their fertile window for conception or contraception, and recognize abnormalities that might require medical attention. While a 28-day cycle is often cited as standard, "normal" encompasses a wide range, and what matters most is consistency and regularity for each individual woman.