The usual duration of a regular menstrual cycle, as well as the volume of discharge, can vary from cycle to cycle. This shouldn’t be tagged as pathology since these fluctuations are caused by the following factors:

1. Genetic background inherited from your mother. 
2.  
3. Body constitution
4. Nutritional habits
5. Climatic environmental conditions

Teenagers might often have irregular cycles during the first year after the onset of their menarche, as it needs time to establish and can vary in duration.  

Occurrence of menstruation after childbirth may take a year or a little more. For 4-6 months after the first menstruation in the postpartum period, all the cycle parameters may fluctuate.

The menstrual cycle also changes in the premenopausal period.