Menstruation is the monthly shedding of the endometrial lining through the vagina, occurring as part of the menstrual cycle. The average cycle length is 28 days (range 21-35 days in adults), with menstruation lasting 3-7 days.
Menstrual Cycle Phases
Ovarian Cycle
The follicular phase (days 1-14 in a 28-day cycle) is dominated by FSH, which recruits and matures ovarian follicles. Ovulation occurs around day 14, triggered by the LH surge, releasing the mature oocyte from the dominant follicle. The luteal phase (days 14-28) is dominated by progesterone from the corpus luteum.
Uterine (Endometrial) Cycle
The menstrual phase (days 1-5) involves shedding of the functional endometrial layer triggered by estrogen and progesterone withdrawal. The proliferative phase (days 6-14) driven by estrogen involves endometrial regeneration and proliferation. The secretory phase (days 15-28) driven by progesterone involves glandular secretion, stromal edema, and vascularization preparing the endometrium for implantation.
Hormonal Profile
FSH rises in the early follicular phase to drive follicle development. LH is low until a massive surge triggers ovulation. Estradiol rises through the follicular phase to peak just before the LH surge, then declines briefly before a secondary peak in the mid-luteal phase. Progesterone is low through the follicular phase, rises sharply after ovulation, and peaks in the mid-luteal phase. If pregnancy does not occur, the corpus luteum regresses, progesterone and estrogen fall, and menstruation begins.
Common Menstrual Symptoms
Premenstrual Syndrome
PMS affects 50-80% of reproductive-age women (moderate to severe in 20-30%). Symptoms appear 5-7 days before menses (luteal phase) and resolve within 4 days of menses onset. Physical symptoms include breast tenderness, bloating, fatigue, headache, joint/muscle pain, food cravings (sweets, salt), weight gain, and acne. Emotional symptoms include irritability, mood swings, anxiety, depression, and crying spells. Behavioral symptoms include social withdrawal, difficulty concentrating, and sleep disturbance (insomnia or hypersomnia). Management: exercise, stress reduction, limiting caffeine, alcohol, and salt, calcium supplements (1,200 mg/day), and SSRIs for severe symptoms (continuous or luteal phase dosing).
Premenstrual Dysphoric Disorder
PMDD is a severe form of PMS affecting 3-8% of women. Diagnostic criteria require 5 or more symptoms occurring in the week before menses, improving within days of menses onset: affective lability (mood swings), irritability, depression, anxiety, decreased interest, difficulty concentrating, fatigue, appetite changes, sleep changes, feeling overwhelmed, and physical symptoms. Symptoms must cause significant functional impairment. Treatment: SSRIs are first-line (fluoxetine, sertraline, escitalopram — can be continuous or luteal-phase only), cognitive behavioral therapy, and hormonal options (combined oral contraceptives with drospirenone — the only FDA-approved OCP for PMDD).
Menstrual Disorders
Amenorrhea
Primary amenorrhea: no menarche by age 15 (with normal secondary sexual characteristics) or by age 13 (without secondary sexual characteristics). Causes include hypothalamic-pituitary disorders (Kallmann syndrome, functional hypothalamic amenorrhea), ovarian causes (Turner syndrome, premature ovarian insufficiency), outflow tract obstruction (Müllerian agenesis — MRKH syndrome, imperforate hymen), and congenital adrenal hyperplasia. Secondary amenorrhea: absence of menses for 3+ months in a woman with previous normal cycles. The most common cause is pregnancy (always first test). Other causes: functional hypothalamic amenorrhea (stress, weight loss, exercise), polycystic ovary syndrome, premature ovarian insufficiency, hyperprolactinemia, thyroid disorders, and Asherman syndrome (intrauterine adhesions from prior uterine surgery).
Abnormal Uterine Bleeding
The PALM-COEIN classification system for abnormal uterine bleeding includes structural causes (PALM: polyp, adenomyosis, leiomyoma/fibroids, malignancy/hyperplasia) and non-structural causes (COEIN: coagulopathy, ovulatory dysfunction, endometrial, iatrogenic, not yet classified). Heavy menstrual bleeding (menorrhagia): bleeding that interferes with physical, emotional, or social quality of life. Objective criteria: soaking through 1+ pad/tampon per hour for several hours, passing large clots, or bleeding for more than 7 days. Iron deficiency anemia is a common consequence. Treatment: levonorgestrel intrauterine system (Mirena — first-line, reduces menstrual blood loss by 80-90%), tranexamic acid (antifibrinolytic, 650-1,300 mg three times daily during menses — reduces bleeding by 40-60%), NSAIDs (reduce bleeding by 20-30%), combined hormonal contraceptives, and endometrial ablation or hysterectomy for refractory cases.
Summary
The menstrual cycle involves coordinated hormonal changes (FSH, LH, estrogen, progesterone) driving the ovarian and endometrial cycles. PMS and PMDD are common, with SSRIs as first-line pharmacotherapy. Dysmenorrhea is treated with NSAIDs and hormonal contraception. Amenorrhea requires evaluation for pregnancy, hypothalamic causes, PCOS, and ovarian insufficiency. Abnormal uterine bleeding is classified by the PALM-COEIN system; the levonorgestrel IUD is the most effective medical treatment for heavy menstrual bleeding.