During the Perimenopause the pattern of your periods will change, becoming more irregular and erratic. They may also become heavier than previously.
Because the biological processes controlling the hormones which trigger periods developed in a time when few people lived beyond middle age, our fertility starts to reduce after our twenties. Some variation to the menstrual cycle thereafter is not uncommon, although not always noticeable.
However, some women can carry on right up to the point of their Menopause with little change to their cycle, but for others their periods may be erratic for a few years.
The conclusion is that its difficult to judge exactly when a change to the menstrual cycle means you’ve entered into the Perimenopause.