Body & health

Menstrual cycle

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Not yet reviewed by a clinician. This health entry was written from the public-health sources listed below and is general information only. It is not medical advice.

Definition

The menstrual cycle is the roughly monthly sequence of hormonal changes that prepares the body for a possible pregnancy, counted from the first day of one period to the day before the next.

At a glance

  • The cycle is counted from the first day of one period to the day before the next.
  • Rising estrogen in the first half helps an egg mature and thickens the uterine lining.
  • A surge of luteinising hormone around the middle triggers ovulation.
  • An average cycle is often quoted as 28 days, but about 21 to 35 days is common in adults.
  • Hormonal contraception changes or pauses the natural cycle.

In detail

In the first half of the cycle, rising estrogen helps an egg mature and thickens the uterine lining. Around the middle, a surge of luteinising hormone triggers ovulation. Afterwards, progesterone keeps the lining ready for implantation. If no pregnancy occurs, hormone levels fall and the lining is shed as a period.

An average cycle is often quoted as 28 days, but anything from about 21 to 35 days is common in adults. Tracking cycles can help people understand their energy, mood and symptoms, and spot changes worth mentioning to a clinician. Hormonal contraception changes or pauses the natural cycle.

In a sentence

  • I started tracking my menstrual cycle in an app to see how it affects my energy.
  • The nurse asked how long her menstrual cycle usually is.

Where the word comes from

Menstrual comes from Latin menstruus, meaning monthly.

Common questions

How do I count the length of my cycle?

Count from the first day of one period up to the day before the next period starts. Anything from about 21 to 35 days is common in adults, and cycles often vary a little from month to month.

Why track the menstrual cycle?

Tracking can help people understand patterns in energy, mood and symptoms and notice changes worth mentioning to a clinician. It is also used in fertility awareness, though tracking alone is not a reliable form of contraception for everyone; a clinician or contraception service can explain the options.

Related terms

Sources

  1. NHSFertility in the menstrual cycle
  2. Planned ParenthoodMenstruation