13 y/o daughter has very irregular periods&this time it been extremely light mostly brown/pink& today will make it 12 days and possibly more. cancer??

Normal: It is highly normal for a 13 year old to have irregular periods in regards to timing and their length. Cancer in this age group exceedingly rare.

Not unusual: In the beginning many young women do not ovulate (release eggs) with their cycles. Ovulation enables a hormone to form that helps pace the system. After 6mo to a year most are ovulating and patterns normalize. There are a subgroup of women that will never have predictable cycles. It would be good to discuss your concerns with your doc. It is common and certainly more likely than a very rare cancer