What is a menstrual cycle?

Each month, the body goes through a number of hormone-driven changes to prepare for a possible pregnancy. This series of events is known as the menstrual cycle.

The menstrual cycle in a nutshell

During each cycle, an egg is developed and released from the ovaries and the lining of the uterus builds up. If a pregnancy doesn’t happen, the lining sheds during the period and the cycle starts again.

This means that a cycle is counted from the first day of one period up to the first day of the next period.


Period 1: 15 – 21 July
Period 2: 14 – 19 August
Cycle length: 30 days

To calculate my cycle length, I count from 15 July to 13 August. My second period started on 14 August, so this is what I count as day one, which makes it a 30-day cycle.

What is a ‘normal’ cycle?

A period that happens every 21 to 35 days is considered a typical regular cycle, but everyone’s normal is different. It’s all about figuring out what your normal is!

Most periods last 3 – 5 days on average, but anywhere from 2 – 7 is usual.

Phases of the menstrual cycle

A menstrual cycle is more than just a period, it can actually be divided into four phases:

  1. Menstrual phase
  2. Follicular phase
  3. Ovulation phase
  4. Luteal phase

When should I see a doctor?

If you haven’t had your first period by the age of 16, it is recommended you see a doctor [1]. You should also see a doctor if your periods were once regular and have become irregular [2].

If you are bleeding heavily and longer for 7 days, this is not normal. Excruciating period pain is another thing you should always see a doctor about.

If you are bleeding outside of period, make a note of what cycle day it is and contact your doctor.

Further reading:


  1. Delayed periods, nhs.uk
  2. When Your Period Signals A Problem, web.md