The Menstrual Cycle

This lesson covers: 

  1. The changes that occur during the menstrual cycle
  2. Why the menstrual cycle occurs

The phases of the menstrual cycle

Starting at puberty, most females experience a menstrual cycle.


The menstrual cycle lasts for around 28 days and is divided into four phases.

Diagram showing the phases of the menstrual cycle including menstruation, uterus lining thickening, ovulation, and maintenance.

Phase 1: Menstruation

  • Day 1-5 - The uterus lining breaks down and is shed through the vagina. This is often called a ‘period’.


Phase 2: Uterus lining thickens

  • Day 5-13 - The uterus lining build ups and thickens.


Phase 3: Ovulation

  • Day 14 - The ovary releases an egg cell, this is the time when the female is most fertile.


Phase 4: Maintenance

  • Day 15-28 - The uterus lining remains thick in case an egg cell is fertilised. If the egg cell is not fertilised the cycle starts again.


Note: The length of these phases can differ between individuals.

The purpose of the menstrual cycle

The menstrual cycle happens to prepare the body for a potential pregnancy each month.

Graph showing the thickness of the uterus lining during the menstrual cycle, including stages of preparation for a fertilised egg and cycle restart.

The thickness of the uterus lining changes so that it can receive a fertilised egg.


If there is no fertilised egg, the cycle starts again, and menstruation occurs. This provides another chance for fertilisation to occur.


The average menstrual cycle lasts for 28 days but there are variations in timing depending on the individual.