Pregnancy

• If you are pregnant, one of the first signs may be a late period.
  
  

Medical Conditions

• Some medical conditions that can cause late or irregular periods include polycystic ovarian syndrome, hormonal imbalances, premature ovarian failure or pituitary diseases.
  
  

Stress or Illness

• Stress, illness or even changes in routine may cause a period to be late or irregular.
  
  

Weight Changes

• Sudden weight loss or gain can cause changes in the menstrual cycle.
  
  

Medication

• Changes in medication, or use of birth control or other contraception, may cause late or irregular menstruation.
  
  

Life Cycle Changes

• When a young girl first starts having her period, it will often be irregular. During the first two years of menstruation, it is normal for a girl to have irregular or missed periods. Likewise, when a woman is nearing menopause, her periods may become irregular. Her menstrual cycle may come to a sudden stop, or slow down gradually.