Every number, and where it came from.

Menstrual cycle parameters

1. Munro MG, Critchley HOD, Fraser IS; FIGO Menstrual Disorders Committee. 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;143(3):393–408.From tool Primary source for all FIGO System 1 thresholds used on this site: frequency 24–38 days, duration 8 days or fewer, and the age-banded regularity criteria.
2. Jain V, Munro MG, Critchley HOD. Contemporary evaluation of women and girls with abnormal uterine bleeding: FIGO Systems 1 and 2. Int J Gynecol Obstet. 2023;162 Suppl 2:29–42.From tool 2023 update confirming the System 1 parameters and the PALM-COEIN classification of causes.
3. Munro MG, Balen AH, Cho S, et al. The FIGO ovulatory disorders classification system. Int J Gynecol Obstet. 2022;159:1–20.From tool Defines the ovulatory disorder spectrum and how it presents as menstrual change — the basis for the point that irregular bleeding follows irregular ovulation.
4. American College of Obstetricians and Gynecologists. Committee Opinion No. 651: Menstruation in girls and adolescents: using the menstrual cycle as a vital sign. Obstet Gynecol. 2015;126(6):e143–e146.From tool Establishes the menstrual cycle as a vital sign, and the age-specific expectations for the years after menarche.
5. Bull JR, Rowland SP, Berglund Scherwitzl E, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med. 2019;2:83.From tool Source for the population mean cycle length of approximately 29.3 days and the spread around it.
6. Harlow SD, Lin X, Ho MJ. Analysis of menstrual diary data across the reproductive life span. J Clin Epidemiol. 2000;53:722–733.From tool Within-woman cycle variability by age — the evidence behind treating variability as age-dependent rather than fixed.

Ovulation as the organising mechanism

7. Nguyen JD, Duong H. Anovulatory bleeding. In: StatPearls. Treasure Island (FL): StatPearls Publishing; updated 2025.Re-verify Supports the core mechanism used throughout this site: without ovulation no progesterone is produced, leaving unopposed estrogen stimulation of the endometrium and irregular, prolonged and often heavy bleeding. Author attribution and current revision date taken from the NCBI Bookshelf record and should be confirmed; StatPearls chapters are revised and re-attributed over time.
8. Speroff L, Fritz MA. Clinical Gynecologic Endocrinology and Infertility. Philadelphia: Lippincott Williams & Wilkins.Re-verify Origin of the four-category classification of dysfunctional bleeding used on this site — estrogen withdrawal, estrogen breakthrough, progesterone withdrawal and progesterone breakthrough bleeding. The framework is reproduced widely in the secondary literature and in clinical reviews; the specific edition, year and pagination were not verified in this build and must be supplied before formal citation.

Menarche and adolescence

9. American College of Obstetricians and Gynecologists' Committee on Adolescent Health Care. Committee Opinion No. 651: Menstruation in girls and adolescents: using the menstrual cycle as a vital sign. Obstet Gynecol. 2015;126(6):e143–e146. doi:10.1097/AOG.0000000000001215Verified Endorsed by the American Academy of Pediatrics. Source of every adolescent parameter used on this site: median age at menarche between 12 and 13 years (median 12.43); 98% have started by 15, within 3 years of thelarche; threshold moves to 14 with hirsutism, disordered eating or excessive exercise; most bleed 2–7 days at the first menses; 90% of early cycles fall between 21 and 45 days; by the third year 60–80% are 21–34 days; and an interval of 90 days or more is the 95th percentile and uncommon even in adolescence. Also the origin of the menstrual-cycle-as-vital-sign framing.
10. Wang Z, Asokan G, Onnela JP, et al. Menarche and time to cycle regularity among individuals born between 1950 and 2005 in the US. JAMA Netw Open. 2024;7(5):e2412854. doi:10.1001/jamanetworkopen.2024.12854Verified Apple Women's Health Study, 71,341 US participants. Mean age at menarche fell from 12.5 years (born 1950–1969) to 11.9 years (born 2000–2005); early menarche before age 11 rose from 8.6% to 15.5% and very early menarche before age 9 from 0.6% to 1.4%; the proportion reaching cycle regularity within two years fell from 76% to 56%. In a subset providing BMI at menarche, roughly 46% of the decline was attributable to higher BMI. Self-recalled age in a non-population-representative app cohort — reported here alongside the ACOG statement rather than in place of it. Author list abbreviated; verify full order before formal citation.

Heavy menstrual bleeding

11. National Institute for Health and Care Excellence. Heavy menstrual bleeding: assessment and management. NICE guideline NG88. London: NICE; 2018 (updated 2021).Re-verify Source of the quality-of-life definition subsequently adopted by FIGO and ACOG. Confirm the current update date before publication.
12. Centers for Disease Control and Prevention. Heavy menstrual bleeding. Atlanta: CDC.Re-verify Source of the duration-over-7-days and change-more-often-than-2-hours criteria. Confirm the current page title and revision date.
13. DeLoughery E, Colwill AC, Edelman A, Bannow BS. Red blood cell capacity of modern menstrual products: considerations for assessing heavy menstrual bleeding. BMJ Sex Reprod Health. 2024;50(1):21–26.From tool Measured red-cell capacities for pads, tampons, cups, discs and period underwear — the basis for volume estimation in the blood-loss tool, and for the point that marketed capacity is not red-cell capacity.
14. Warner PE, Critchley HOD, Lumsden MA, Campbell-Brown M, Douglas A, Murray GD. Menorrhagia I: measured blood loss, clinical features, and outcome in women with heavy periods: a survey with follow-up data. Am J Obstet Gynecol. 2004;190(5):1216–1223.From tool Documents the divergence between measured volume and reported experience of heaviness.

Perimenopause and menopause

15. Harlow SD, Gass M, Hall JE, et al. Executive summary of the Stages of Reproductive Aging Workshop +10: addressing the unfinished agenda of staging reproductive aging. J Clin Endocrinol Metab. 2012;97(4):1159–1168.Re-verify The STRAW+10 staging framework applied by the Perimenopause Stage Finder. Confirm volume, issue and pagination — this citation also appeared in Menopause and Climacteric in 2012, and the versions differ.
16. The 2022 Hormone Therapy Position Statement of The North American Menopause Society Advisory Panel. The 2022 hormone therapy position statement of The North American Menopause Society. Menopause. 2022;29(7):767–794.From tool Candidacy criteria, contraindications, route-of-administration risk differences, progestogen comparison, and the timing-window recommendation.
17. Shufelt CL, Brown V, Carpenter JS, et al; NAMS 2023 Nonhormone Therapy Position Statement Advisory Panel. The 2023 nonhormone therapy position statement of The North American Menopause Society. Menopause. 2023;30(6):573–590.From tool Evidence grades and effect sizes for non-hormonal treatment of vasomotor symptoms.
18. Manson JE, Chlebowski RT, Stefanick ML, et al. Menopausal hormone therapy and health outcomes during the intervention and extended poststopping phases of the Women's Health Initiative randomized trials. JAMA. 2013;310(13):1353–1368.From tool Extended WHI follow-up with outcomes stratified by age and timing of initiation — the basis for the statement that the original findings were misapplied to younger women.
19. Panay N, Fenton A, Hamoda H, et al; IMS Recommendations Writing Group. International Menopause Society recommendations and key messages on women's midlife health and menopause. Climacteric. 2025;28(6):634–656.From tool GRADE-based recommendations applied by the HRT Candidacy & Timing Navigator.

Abnormalities — classification and criteria

20. Munro MG, Balen AH, Cho S, Critchley HOD, Díaz I, Ferriani R, et al; FIGO Committee on Menstrual Disorders and Related Health Impacts, and FIGO Committee on Reproductive Medicine, Endocrinology, and Infertility. The FIGO ovulatory disorders classification system. Int J Gynecol Obstet. 2022;159(1):1–20. doi:10.1002/ijgo.14331Verified Source of the HyPO-P classification (Type I hypothalamic, II pituitary, III ovarian, IV PCOS) and the second-level GAIN-FIT-PIE subclassification. Replaces the WHO ovulatory disorder groups.
21. Teede HJ, Tay CT, Laven JJE, Dokras A, Moran LJ, Piltonen TT, et al. Recommendations from the 2023 international evidence-based guideline for the assessment and management of polycystic ovary syndrome. J Clin Endocrinol Metab. 2023;108(10):2447–2469.Verified Source for the 2023 PCOS criteria: AMH permitted as an alternative to ultrasound in adults only; ultrasound and AMH not recommended within 8 years of menarche; neither required where irregular cycles and hyperandrogenism coexist. Author list and pagination confirmed against the journal record; verify full author order before formal citation.
22. European Society of Human Reproduction and Embryology (ESHRE) Guideline Group on POI. Management of women with premature ovarian insufficiency. Hum Reprod. 2016;31(5):926–937.Re-verify Source of the POI diagnostic criteria used on this site: oligo/amenorrhea ≥4 months before age 40 with FSH >25 IU/L on two occasions >4 weeks apart. The criteria themselves were confirmed across multiple secondary sources; confirm the guideline citation and whether a later update supersedes it.
23. Clarke MA, Long BJ, Del Mar Morillo A, Arbyn M, Bakkum-Gamez JN, Wentzensen N. Association of endometrial cancer risk with postmenopausal bleeding in women: a systematic review and meta-analysis. JAMA Intern Med. 2018;178(9):1210–1222. doi:10.1001/jamainternmed.2018.2820Verified 129 studies. Pooled prevalence of postmenopausal bleeding among women with endometrial cancer approximately 91%; pooled risk of endometrial cancer among women with postmenopausal bleeding 9% (95% CI 8–11%), ranging from 7% among hormone therapy users to 12% when they were excluded.
24. American College of Obstetricians and Gynecologists. Committee Opinion No. 580: Von Willebrand disease in women. Obstet Gynecol. 2013;122(6):1368–1373.Re-verify Basis for the recommendation to screen adolescents with severe heavy menstrual bleeding. Confirm number, pagination and whether the opinion has since been reaffirmed, withdrawn or replaced.
25. Shankar M, Lee CA, Sabin CA, Economides DL, Kadir RA. Von Willebrand disease in women with menorrhagia: a systematic review. BJOG. 2004;111(7):734–740.Re-verify Source of the pooled von Willebrand disease prevalence of approximately 13% (range across studies 5–24%) among women with heavy menstrual bleeding. The figures were confirmed via secondary citation; verify the primary record directly before publication.

The 2025–2026 FDA label change

26. U.S. Department of Health and Human Services; U.S. Food and Drug Administration. HHS advances women's health, removes misleading FDA warnings on hormone replacement therapy [news release]. Silver Spring, MD: FDA; 10 November 2025.Verified Announcement that the FDA would initiate removal of the boxed warnings, following a scientific literature review, a July 2025 expert panel and a public comment period.
27. U.S. Food and Drug Administration. FDA approves labeling changes to menopausal hormone therapy products [news release]. Silver Spring, MD: FDA; 12 February 2026.Verified Approval of the first six revised labels, removing the cardiovascular disease, breast cancer and probable dementia statements from the boxed warning; 29 manufacturers had submitted proposed changes. The endometrial cancer boxed warning on systemic estrogen-alone products remains.

Where a claim on this site could not be traced to one of the sources above, it is not stated. Where two sources disagree, both are shown rather than reconciled. Corrections are welcome — obmd.com.