Periods · 6 min
How the menstrual cycle actually works
Four phases, shifting hormones, and why “28 days” is a typical average — not a rule.
A loop, not a countdown
The menstrual cycle is the monthly pattern of hormonal change that prepares the body for a possible pregnancy. It is commonly described in four phases: menstrual, follicular, ovulation, and luteal. A “textbook” cycle is 28 days, but public guidance on a typical adult range varies: many clinicians use about 21 to 35 days (ACOG is often cited), while the U.S. Office on Women’s Health describes regular cycles as usually every 24 to 38 days (aligned with FIGO). What matters more than matching a calendar is whether your own pattern is roughly consistent for you — and whether it has newly changed.
The four phases
Menstruation is the shedding of the uterine lining. The follicular phase overlaps with the period and continues as a follicle matures. Ovulation is the release of an egg, usually about two weeks before the next period. The luteal phase follows ovulation; if pregnancy does not occur, hormone levels fall and a period begins. These are patterns, not stopwatches.
Why tracking helps
Logging period start dates, flow, and a few symptoms gives you a personal baseline. That baseline is useful when something changes — a much shorter cycle, newly severe pain, or bleeding that soaks through protection hourly. Tracking is information, not a diagnosis.
Sources
- Office on Women's Health (U.S. Department of Health and Human Services). Your menstrual cycle. womenshealth.gov, 2025.
- National Health Service (UK). Periods and fertility in the menstrual cycle. NHS.
- Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Gemzell Danielsson K, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine, 2019.
- American College of Obstetricians and Gynecologists. Committee Opinion No. 651: Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign. Obstetrics & Gynecology / ACOG, 2015.
Educational information for general awareness. This article has not been individually reviewed by a clinician for you, and it is not medical advice.