Menstrual cycle
The time from the first day of one period to the day before the next.
What does menstrual cycle mean?
Menstrual cycle: The time from the first day of one period to the day before the next.
How it is used in the Splash library
Your thyroid makes hormones that set the pace for loads of body systems, including your menstrual cycle.
From “Your thyroid and your cycle”A Menstrual cycle is the full loop your body runs, from the first day of one period to the day before the next one starts.
From “What actually is a menstrual cycle?”The American College of Obstetricians and Gynecologists has called the menstrual cycle a "vital sign", and a 2025 review in The Lancet Obstetrics, Gynaecology and Women's Health made the same case across the whole lifespan.
From “Your cycle is a vital sign”A 2025 systematic review of 11 studies found the evidence was suggestive that ADHD symptoms change with hormones, including across the menstrual cycle.
From “ADHD and your cycle”Running a menstrual cycle takes energy.
From “Eating enough, and your periods”
Menstrual cycle explained
The menstrual cycle is the monthly series of hormone changes that prepares the body for a possible pregnancy. Each cycle starts on the first day of a period and ends the day before the next.
It has two main phases, separated by ovulation: the follicular phase, when an egg matures and the womb lining rebuilds, and the luteal phase, when progesterone prepares the lining for implantation. If there is no pregnancy, hormone levels fall and the lining is shed as a period.
What's typical
A cycle usually lasts 21 to 35 days, and a period 2 to 7 days (NHS). Most of the variation in cycle length comes from the follicular phase.
Periods usually start between ages 10 and 16 and stop at menopause, on average at 51 in the UK. Cycles are often irregular for the first few years and again during perimenopause.
Doctors increasingly treat the cycle as a vital sign: changes can be an early clue to other health issues (ACOG).
When to talk to a GP, nurse or pharmacist
- You haven't started your periods by 16, or by 15 with other signs of puberty.
- Your periods are very heavy, very painful, last longer than 7 days, or stop for 3 months or more.
- You bleed between periods, after sex or after menopause.
In the UK you can call NHS 111 for advice at any time. In an emergency call 999 (UK) or 911 (US). This is general information, not medical advice.
Where it comes up
Sources
Sources checked October 2026.
See it in your own cycle.
Log a few periods and Splash shows your patterns, privately, on your phone. Free, no account.