Short menstrual cycle (20 days) — should I worry?

Note on the name: in May 2026, polycystic ovary syndrome (PCOS) was formally renamed polyendocrine metabolic ovarian syndrome (PMOS), following a global consensus process. The old name was inaccurate — the condition affects multiple hormone and metabolic systems, not just the ovaries, and "polycystic" wrongly implies ovarian cysts. Nothing about the condition, its diagnosis or its treatment has changed. Both names will be used side by side for the next few years, and we use PCOS below since that is the term most people know.

A cycle that is consistently shorter than 24 days is worth investigating. Under FIGO's menstrual terminology, anything below 24 days is classed as frequent menstruation, so a 20-day cycle sits clearly outside the normal range. It may mean you are ovulating earlier than average, or that your luteal phase — the stretch between ovulation and your period — is too short to support an embryo implanting. That second possibility matters particularly if you are trying to conceive. Common causes include thyroid dysfunction, raised prolactin, PCOS, and perimenopause. Please see a gynaecologist. Day 2–3 blood tests for FSH, LH, prolactin and thyroid function, together with a pelvic ultrasound, are usually where the assessment starts. Bring a record of your last few cycles if you have one — it genuinely helps.