Menstrual Cycle: Phases, Hormonal Regulation, Menarche, Menopause and Andropause

Focus: menstrual cycle and reproductive-life stages | Audience: emergency medicine, clinical medicine, nursing, midwifery and allied-health students.

Learning objectives

• Describe the ovarian and uterine phases of the menstrual cycle and their hormonal control.
• Explain menstruation, ovulation, the fertile window, menarche, perimenopause, menopause and male late-onset hypogonadism/“andropause.”
• Recognise abnormal bleeding, amenorrhoea and dysmenorrhoea, and identify emergency presentations.

1. Definitions and normal variation

Menstruation is cyclical shedding of the endometrial functional layer through the vagina. The menstrual cycle is the coordinated sequence of ovarian follicle development, ovulation, corpus-luteum activity and endometrial preparation. Day 1 is the first day of true menstrual bleeding, not the last day of spotting.

The supplied Slideshare deck describes an average cycle of about 28 days, usually 25–36 days, with bleeding around five days and an average blood loss around 30 mL. These figures are teaching averages, not a requirement for every person. Cycle length varies because the follicular phase is more variable; the luteal phase is usually more stable.

2. The hormones and feedback loops

Low ovarian steroids after corpus-luteum regression reduce negative feedback, allowing FSH to rise and recruit the next cohort of follicles. Sustained high oestradiol from a dominant follicle reverses feedback to positive, producing an LH surge.

3. The four phases of a typical cycle

4. Menstrual phase: what causes bleeding?

1. In the absence of pregnancy, hCG is absent and the corpus luteum involutes.
2. Progesterone and oestradiol fall, causing spiral-artery vasoconstriction, tissue ischaemia and inflammatory mediator/prostaglandin release.
3. The superficial functional layer breaks down and is expelled with blood, mucus and tissue through the cervix and vagina.
4. The basal layer remains to regenerate the endometrium during the next proliferative phase.

Menstrual blood loss is affected by uterine contractility, coagulation, fibroids, adenomyosis, medication, endocrine disorders and pregnancy-related pathology—not only by “hormone imbalance.”

5. Follicular and proliferative phase

• FSH recruits a cohort of antral follicles. Granulosa cells convert theca-derived androgens to oestradiol.
• Oestradiol initially suppresses FSH; the follicle with the best sensitivity becomes dominant while others undergo atresia.
• Endometrium regenerates from basalis, glands lengthen and spiral arteries develop; typical thickness depends on timing and imaging method.
• Cervical mucus becomes increasingly watery, elastic and sperm-friendly near ovulation.

6. Ovulation and the fertile window

• Sustained high oestradiol from the dominant follicle creates positive feedback at hypothalamus/pituitary, generating a large LH surge and smaller FSH surge.
• LH promotes completion of oocyte meiosis I, enzymatic weakening of follicle wall and release of the secondary oocyte surrounded by cumulus cells.
• Fimbriae sweep the oocyte into the tube. Fertilisation, if it occurs, usually takes place in the ampulla.
• Ovulation is not reliably day 14 in every cycle. It generally occurs about 14 days before the next period, so a longer/shorter cycle shifts the fertile window.
• Clues include clear “egg-white” cervical mucus, mild mittelschmerz, LH urine surge and a sustained small rise in basal body temperature after ovulation. None is perfect alone.

7. Luteal/secretory phase

• The ruptured follicle becomes the corpus luteum and secretes progesterone, oestradiol and inhibin A.
• Progesterone reduces GnRH/LH/FSH, makes cervical mucus thick and converts proliferative endometrium to secretory glands filled with glycogen.
• If implantation occurs, trophoblast hCG rescues the corpus luteum until placental steroid production is sufficient.
• If not, corpus luteum regresses to corpus albicans; steroids fall, prostaglandins rise and a new menstrual phase begins.

8. Menarche and adolescence

Menarche is the first menstrual period and signals that the HPG axis has become active, but early cycles are often anovulatory and irregular. Puberty also includes thelarche, pubarche, growth acceleration, body-composition change and emotional development.

• Teach menstrual tracking, hygiene, expected variation, pain management, nutrition, iron intake, contraception, STI prevention and consent.
• Assess heavy bleeding for anaemia and inherited bleeding disorders, especially if bleeding began from menarche, causes flooding or there is easy bruising/family history.
• Evaluate delayed puberty, primary amenorrhoea, pregnancy, eating disorder, excessive exercise, chronic illness, endocrine disease and genital outflow obstruction when indicated.
• Irregular cycles do not protect against pregnancy; ovulation can precede the first remembered period.

9. Menopause and the menopausal transition

Perimenopause is the transition with changing follicle activity, irregular cycles and fluctuating oestradiol. Menopause is diagnosed retrospectively after 12 consecutive months without menstruation when no other cause explains amenorrhoea. Postmenopause is the period afterward.

Common symptoms

• Hot flushes/night sweats, sleep disturbance, mood change, vaginal dryness, dyspareunia, urinary symptoms and reduced libido.
• Long-term oestrogen loss is associated with bone loss and changes in cardiovascular risk.
• Bleeding after 12 months of amenorrhoea is not “normal menopause”; investigate for endometrial, cervical, vaginal or other causes.

Clinical care

• Take menstrual, pregnancy, medication, cancer, thromboembolism, migraine, liver and cardiovascular history; assess blood pressure, BMI, bleeding, mood and bone risk.
• Discuss lifestyle, exercise/resistance training, calcium/vitamin D adequacy, smoking cessation, sleep, sexual health and treatment choices.
• Menopausal hormone therapy, local vaginal oestrogen or non-hormonal options require individual risk assessment and local guideline review; do not prescribe solely from a single FSH value in typical age groups.

10. Andropause and late-onset hypogonadism

Andropause is a popular but misleading term because testosterone decline is gradual and does not represent a universal abrupt cessation like menopause. Prefer late-onset hypogonadism/testosterone deficiency when a person has compatible symptoms plus consistently low morning testosterone on appropriately repeated testing, after considering obesity, diabetes, sleep apnoea, opioids, glucocorticoids, pituitary/testicular disease and medication effects.

• Possible symptoms include reduced libido, erectile dysfunction, fewer spontaneous erections, fatigue, low mood, reduced muscle mass, anaemia and low bone density—but these are non-specific.
• Investigate with morning total testosterone on two occasions; add SHBG/free testosterone, LH/FSH, prolactin, thyroid and pituitary tests when indicated.
• Testosterone therapy is not a general anti-ageing treatment and may suppress spermatogenesis, raise haematocrit, worsen untreated OSA and require prostate/cardiovascular monitoring. Refer to endocrinology/urology before treatment.

11. Abnormal menstrual patterns

12. Clinical assessment

1. Clarify age, pregnancy possibility, LMP, cycle interval, duration, volume (pads/tampons/clots/flooding), pain, sexual history, contraception and postpartum/breastfeeding status.
2. Ask red flags: syncope, shoulder-tip pain, unilateral severe pain, fever, offensive discharge, pregnancy, anticoagulants, bleeding disorder, weight change, galactorrhoea, virilisation and postmenopausal bleeding.
3. Check vitals, pallor, hydration, BMI, thyroid, abdomen and pelvic examination when indicated and consented.
4. Investigate with pregnancy test, CBC/ferritin and targeted STI, thyroid, prolactin, coagulation, ultrasound or other tests guided by findings.
5. Document the patient’s goals: symptom relief, fertility, contraception, pregnancy avoidance or menopause support.

13. Emergency presentations

14. Supportive and preventive care

• Promote menstrual health education, affordable products, hand hygiene and safe disposal; menstruation is not dirty or a reason for exclusion.
• Address iron deficiency, balanced nutrition, exercise, sleep, stress, chronic illness and reproductive coercion.
• Provide contraception and STI counselling according to informed preference, not assumptions about marital status.
• Use culturally respectful language while correcting harmful myths, forced pregnancy or unsafe remedies.

15. Quick self-test

1. What is day 1 of the menstrual cycle?
2. Which phase varies most in length and why?
3. Explain the LH surge and ovulation.
4. What does progesterone do in the luteal phase?
5. Differentiate menarche, perimenopause, menopause and postmenopause.
6. Why is “andropause” an imprecise term?
7. List five emergency symptoms in a patient with pelvic pain or bleeding.
8. What history is essential before interpreting abnormal bleeding?

Further study and references

Starting source: Menstrual cycle. The post expands its 37-slide coverage of phases, hormones, amenorrhoea, dysmenorrhoea, menorrhagia and related bleeding patterns. Also consult current ACOG/WHO guidance on menstruation and menopause and Ugandan reproductive-health protocols.