The normal menstrual cycle & HPO axis

You are one diagram away from owning gynaecology. Draw the HPO axis with the four hormone curves and the follicular/luteal phases, and the whole of menstrual medicine — polycystic ovary syndrome, amenorrhoea, heavy and irregular bleeding, ovulation induction — stops being a list of diseases and becomes a set of places the axis can break. This topic is the foundation: read it once, then come back to it every time a later topic says "because oestrogen was unopposed".^[1]

Overview and definition

The menstrual cycle is the number of days from the first day of one menses to the day before the next. In eumenorrhoeic women it is a remarkably regular oscillation: the median is about 28 days, with most cycles falling between 25 and 30 days; across larger studies the normal range spans 24 to 38 days.^[1][2][12]

Two clocks run in parallel and you should name both:^[1]

• The ovarian cycle — follicular phase (menses to ovulation) then luteal phase (ovulation to next menses).
• The endometrial cycle — proliferative phase (oestrogen-driven growth), secretory phase (progesterone-driven transformation), then menstruation.^[1]

One number anchors the whole topic: the luteal phase is nearly constant at about 14 days. Cycle-length variability lives almost entirely in the follicular phase, which ranges from roughly 10 to 16 days. A woman with a 35-day cycle does not have a "long luteal phase" — she has a long follicular phase.^[1]

Typical menstrual blood loss is about 30 mL; the classical threshold for abnormally heavy loss was over 80 mL. Modern FIGO terminology has retired the word "menorrhagia" in favour of patient-determined "heavy menstrual bleeding", recognising that the 80 mL line never matched how women suffer.^[1][12]

References12ShowHide

1. [1]Reed BG, Carr BR The Normal Menstrual Cycle and the Control of Ovulation. Endotext, 2000.PMID 25905282
2. [2]Mihm M, Gangooly S, Muttukrishna S The normal menstrual cycle in women. Anim Reprod Sci, 2011.PMID 20869180
3. [3]Marsh EE, Shaw ND, Klingman KM, et al. Estrogen levels are higher across the menstrual cycle in African-American women compared with Caucasian women. J Clin Endocrinol Metab, 2011.PMID 21849524
4. [4]Vanden Brink H, Robertson DM, Lim H, et al. Associations Between Antral Ovarian Follicle Dynamics and Hormone Production Throughout the Menstrual Cycle as Women Age. J Clin Endocrinol Metab, 2015.PMID 26465392
5. [5]Robertson DM, Lee CH, Baerwald A, et al. Interrelationships among reproductive hormones and antral follicle count in human menstrual cycles. Endocr Connect, 2016.PMID 27856496
6. [6]Robertson DM, Hale GE, Fraser IS, et al. Changes in serum antimüllerian hormone levels across the ovulatory menstrual cycle in late reproductive age. Menopause, 2011.PMID 21178792
7. [7]Sun BZ, Kangarloo T, Adams JM, et al. Healthy Post-Menarchal Adolescent Girls Demonstrate Multi-Level Reproductive Axis Immaturity. J Clin Endocrinol Metab, 2019.PMID 30289507
8. [8]Shaw ND, Seminara SB, Welt CK, et al. Expanding the phenotype and genotype of female GnRH deficiency. J Clin Endocrinol Metab, 2011.PMID 21209029
9. [9]Lie Fong S, Schipper I, Valkenburg O, et al. The role of anti-Müllerian hormone in the classification of anovulatory infertility. Eur J Obstet Gynecol Reprod Biol, 2015.PMID 25666342
10. [10]Munro MG, Balen AH, Cho S, et al. The FIGO ovulatory disorders classification system. Int J Gynaecol Obstet, 2022.PMID 35983674
11. [11]El Hachem H, Crepaux V, May-Panloup P, et al. Recurrent pregnancy loss: current perspectives. Int J Womens Health, 2017.PMID 28553146
12. [12]Munro MG, Critchley HOD, Fraser IS The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. Int J Gynaecol Obstet, 2018.PMID 30198563