Reasoning GP · Algorithm · 1-page summary
Irregular periods (oligomenorrhoea / AUB)
For: women of reproductive age with irregular, infrequent, frequent or intermenstrual/postcoital bleeding, in primary care. Not for: known pregnancy (bleeding in pregnancy pathway) or expected bleeding on HRT (BMS/NICE HRT pathway). Postmenopausal bleeding is kept separate — block 2.
Core rule: urine hCG first in anyone who could be pregnant; then exclude cervical and endometrial cancer — look at the cervix for postcoital or intermenstrual bleeding, and treat any postmenopausal bleeding as endometrial cancer until proven otherwise. Only then settle on a benign label, and document it.
0Stop — call 999 now — before any history
- Heavy bleeding with dizziness, syncope, tachycardia or hypotension, or with chest pain, breathlessness or collapse → 999 (haemorrhage).
- Possible pregnancy with shoulder-tip pain, collapse or peritonism, or haemodynamic instability → 999 / A&E (ruptured ectopic).
1Triage by urine hCG — if pregnancy possible (NG126)
| + unstable | Instability, significant pain/bleeding, shoulder-tip pain, syncope, peritonism → A&E / 999 |
| + pain | Pelvic pain, tenderness or cervical motion tenderness → Immediate EPAU / OOH gynae |
| + bleeding | Pain, ≥6 weeks or unsure dates → EPAU by status. Else (<6 weeks, no risk factors) → assess ± expectant, safety-net. |
| Negative | Screen (block 2). Recheck hCG if cycles change. |
2Screen — one row is enough
- Postmenopausal bleeding, unexplained, not due to HRT: 55+ → Suspected cancer pathway; under 55 → consider it.
- Cervix looks cancerous → consider Suspected cancer pathway. Persistent PCB/IMB, normal cervix, STI tests negative → Urgent gynae
- Endometrial risk: infrequent heavy bleeding with obesity or PCOS, tamoxifen, failed treatment, pelvic mass, thick endometrium → Urgent gynae
- Persistent bloating, early satiety, pelvic pain: ≥40 CA125 + urgent USS; ≤39 consider urgent USS. Ascites, or mass not obviously fibroids → Suspected cancer pathway
- Visual-field loss, neuro deficit or severe headache + raised prolactin/amenorrhoea → Same-day specialist. Rapid virilisation → Urgent endocrinology
- Low weight + amenorrhoea: suspected eating disorder → Refer without delay; bradycardia, hypotension, syncope, electrolyte upset → Same-day acute medical. Not BMI alone.
3Assess — once blocks 0–2 are answered
☐ Bleeding diary ~3 cycles; contraception, drugs, sexual history, fertility wishes ☐ BMI, BP, hirsutism, acne; visual fields if prolactin raised ☐ Speculum + chlamydia/gonorrhoea NAAT for PCB or IMB; bimanual if pain, mass or CMT ☐ hCG, TSH, prolactin as indicated; FBC if heavy; PCOS: testosterone + FAI/SHBG, HbA1c, lipids; FSH ×2, 4–6 weeks apart, only if POI suspected
Not in primary care: blind endometrial biopsy, random "hormone profiles", pituitary MRI before prolactin confirmed.
4Classify — only if the screen is negative
Infrequent: cycles >35 days, <8 a year or any >90 days. Frequent: <21 days. Irregular in year 1 after menarche is normal.
- PCOS — 2 of 3 after excluding others: ovulatory dysfunction, hyperandrogenism, polycystic ovaries. Cycles + hyperandrogenism → no USS.
- Thyroid / prolactin — repeat a raised prolactin; review antipsychotics, metoclopramide, domperidone, opioids; macroprolactin.
- POI — under 40, menopausal symptoms, raised FSH on 2 samples, never one. Perimenopause — 45+: clinical, no FSH; still needs contraception.
- Hypothalamic — low weight, restriction, exercise, stress; low/normal FSH. Exclusion diagnosis.
- Structural / cervical — IMB, PCB, heavy, enlarged uterus → TVUSS or outpatient hysteroscopy.
- Iatrogenic — hormonal contraception (common first 3 months), anticoagulants, tamoxifen.
- Uncertain — say so; name who reviews and when.
5Treat today — doses: NICE/SPC, BNF, local formulary
- PCOS, no pregnancy wish: combined hormonal contraception 1st line; check UKMEC. Weight loss 5–10% if overweight.
- Prolonged anovulation: after excluding pregnancy, protect the endometrium — CHC, LNG-IUS or intermittent progestogen (bleed at least every 3–4 months).
- Trying to conceive: folic acid, weight optimisation; fertility referral by duration, age and risk.
- Hypothyroid: levothyroxine per NG145. Drug prolactin: change only with the prescriber; never stop an antipsychotic abruptly.
6Refer routinely — and follow up
- Confirmed POI → specialist service (HRT or CHC to at least average menopause age). Unexplained high prolactin → endocrinology. Fibroids, polyps → gynaecology.
- Review: results 1–2 weeks; 3 months — diary, endometrial protection; PCOS yearly — weight, BP, glycaemia, lipids, mood.
7Never reassure by
- A normal cervical screening result — it does not explain PCB.
- A withdrawal bleed — abnormal bleeding still needs investigation.
- Blaming contraception before pregnancy, STI and cervix are checked.
- A normal CA125, or CA125 alone under 40.
- A single FSH or a single prolactin result.
- "Stress" or PCOS under 40 before POI is considered.
8Safety-net — say it, then document it
999 “If you ever feel faint with heavy bleeding, or have strong tummy pain and could be pregnant, call 999 or go to A&E.” Same day: positive pregnancy test with pain or bleeding; new visual loss, neurological symptoms or severe headache. Book now, don't wait for review: “bleeding after sex, between periods, or at all once periods have stopped for a year”; heavier or continuous bleeding, a new pelvic mass, severe pelvic pain or bloating, rapidly progressive virilisation.
Document: hCG result, speculum findings, red flags asked, endometrial-protection plan and safety-net given.
✓Safe to leave primary care only if — all five, documented
Pregnancy excluded; no block 0–2 row applies
Speculum and swabs done if PCB or IMB
One cause explains it, or uncertainty stated
Endometrial protection planned; review owner and date set
Safety-net given and documented
Summary of the full Reasoning GP irregular periods algorithm, which holds the detail, examination and evidence. Sources: NICE NG12 (updated April 2026), NG23, NG88, NG126, NG145, NG257, NG69; International PCOS Guideline 2023; RCOG Green-top 33; CoSRH CEU problematic bleeding 2015; local referral pathways. Clinical reference — follow local policies and check doses: NICE/SPC, BNF.