The reactivation of the hypothalamic-pituitary-gonadal axis, as well as the growth axis, is the most significant shift. The adrenal and thyroid glands have previously undergone maturational changes. The levels of estrogen and LH (luteinizing hormone) do not start to rise until 9-12 years of age. Socioeconomic level, geographic origin, exposure to substances or other environmental factors, genetic effects, psychological factors, and physical activity all influence pubertal development and menarche onset.
The regular menstrual cycle is the product of the hypothalamus, pituitary, ovaries, and uterus interacting: it is a complicated link between hormonal secretion and physiological activities that prepare the body for a future pregnancy. The ovarian cycle and the endometrial cycle are distinct. The ovarian cycle has two phases: follicular and luteal, while the endometrial cycle has three phases: proliferative, secretory, and desquamation (menstruation).
The follicular phase lasts from the first day of the cycle (the first day of the period) to the fourteenth day of the cycle, albeit this duration can vary, and this variability is what causes monthly abnormalities. It necessitates the pulsatile but sustained release of hypothalamic GnRH (gonadotropin-releasing hormone), which stimulates and regulates FSH (follicle stimulating hormone) and LH secretion in the pituitary gland. Increased FSH and hormonal feedback promote the formation of primordial follicles and an increase in E2 by ovarian granulosa cells. This raises the level of LH, and a dominant follicle emerges in the middle of the cycle to mature and prepare for ovulation. Under the trophic impacts of estrogen, the endometrium begins its proliferative phase with an increase in the thickness of its arteries, stroma, and glandular structures during this period.
Around day 14, ovulation occurs after 34-36 hours of peak LH secretion, followed by atresia of the other follicles and evacuation of the egg from the dominant follicle. The corpus luteum, responsible for estrogen and progesterone production, begins to form during the next three days.
The luteal phase is the period between ovulation and the start of menstruation. Because of the increased amounts of E2 and progesterone, LH and FSH release is drastically reduced. The endometrium enters its secretory phase, during which it thickens, experiences spiral artery vascular growth, grows its glandular structure, and matures its stroma. If there is no pregnancy, the corpus luteum atrophies after 10-14 days. This will decrease ovarian hormones (E2 and progesterone) and promote GnRH, FSH, and LH release in the hypothalamus and pituitary, kicking off a new ovarian and endometrial cycle.
Menstruation is the normal monthly shedding of the endometrial mucus as a consequence of hormonal deficiency, and its remains, along with blood, mucus and vaginal cells, are expelled through the vagina. These cyclic variations are crucial because they serve as the basis for an indirect technique for monitoring the endocrine function of the ovary.
The menstrual cycle verifies a young woman's normalcy (in most circumstances) in terms of her future reproductive health and should be considered a vital sign on par with pulse, breathing, and blood pressure.
The average menstrual cycle lasts 28 days, the bleeding period lasts 5 to 8 days, the amount of menstrual fluid produced every cycle ranges from 30 to 80 ml, and the time between menses is 24 to 38 days. Some authors believe that cycles between 21 and 45 days are normal in adolescents, while others believe that this could postpone the detection of a problematic situation.