Cycle basics · 6 minute read
The menstrual cycle, explained
A practical guide to cycle days, changing hormones, bleeding, ovulation, and why a “normal” cycle is a range—not one perfect number.
Evidence-informed · Editorial review August 2026 · Links to trusted sources are available in the interactive app
A cycle is more than a period
Cycle day 1 is the first day of menstrual bleeding. The cycle ends the day before the next period begins. During that time, signals between the brain, ovaries, and uterus coordinate follicle growth, ovulation, and changes to the uterine lining.
There is no universal 28-day clock
Twenty-eight days is a familiar example, not a rule. The U.S. Office on Women’s Health describes regular adult cycles as commonly lasting 24 to 38 days. Length can change with age, stress, illness, travel, medications, pregnancy, breastfeeding, and the transition toward menopause.
What tracking can—and cannot—show
Recording period starts, flow, symptoms, and tests can reveal your own pattern and give a clinician useful context. Calendar estimates are predictions. They cannot prove that ovulation happened and should not be used alone to prevent pregnancy.