Medical conditions such as polycystic ovary syndrome (PCOS) or thyroid problems may cause women to have highly irregular cycles and missed periods. ... If you have missed your period for over 90 days and are not pregnant, talk to your doctor aboutgetting tested for any underlying medical conditions.

Click here to read more

Several outside factors can wreak havoc on your menstrual cycle. Stress, for example, can delay your period. Malnutrition can affect it, too. Your cycle can fluctuate if you drink too much caffeine or don’t eat enough food.

Sudden lifestyle changes, such as intense exercise or working the overnight shift on your job, can also cause your period to be irregular.