The length of a menstrual cycle varies from person to person. The average for all women is about 28 days. Therefore, this is often used as the number to describe a normal menstrual cycle. 28 days is equal to 4 weeks.
Many have a menstrual cycle that differ from 28 days
A menstrual cycle may last somewhere between 21 days and 35 days. Therefore, it is more correct to say that this is a normal menstrual cycle. It is also completely normal if your menstrual cycle varies in length from time to time. It is unusual that the length of the menstrual cycle is exactly the same every time. The length of the cycle often varies several days from time to time.
Then what is an irregular menstrual cycle?
The fact that your menstrual cycle varies several days in length from time to time is common and completely normal. Your menstruation is still considered regular. If you have a regular menstrual cycle, it is easier for you to anticipate when you will have your next period and when during your cycle you will ovulate. If you have a menstrual cycle which varies more than a few days in length, from the shortest menstrual cycle to the longest, your cycle is regarded as irregular.
The number of days that the menstrual cycle can vary before being considered irregular differs. Some claim that if the menstrual cycle varies more than 4 days, it should be regarded as irregular, while others claim that if the menstrual cycle varies more than 7 days (i.e. over a week), it should be considered as irregular. Having an irregular menstrual cycle implies that it is more difficult for you to anticipate when you will have your next period and when, during the cycle, you will ovulate. You can discuss with your midwife how they consider regular and irregular menstruation.
An irregular menstruation can exist without apparent cause. However, there may also be a cause for it. When you are young, under 25 years of age, a menstrual cycle which varies more in length is more common, while when you are older, 25-39 years old, your menstruation is mostly regular. An irregular menstrual cycle can for example be caused if you have an absence of ovulation, if you are breast-feeding, if you have Polycystic Ovary Syndrome or if you are approaching menopause.