Many women will occasionally miss their period for good reason. This may occur if you are pregnant, breastfeeding or have reached menopause. Young women often have irregular periods in the first year or two after first starting their period. If none of these apply to you, the cause may be:

• problems with the pituitary, thalamus or the thyroid gland-these glands secrete hormones which influence the menstrual cycle
• prolonged strenuous exercise
• very low body weight- excess exercise and low body weight (gymnasts are a prime example) may result in absent or irregular periods
• anatomical problems of the cervix, vagina or uterus
• chronic illness- if you have been unwell, you may stop ovulating
• hormonal imbalances
• stress- severe and prolonged stress may actually cause your periods to stop for awhile
• premature menopause- occasionally, women will go into menopause earlier than expected
• ovarian problems- large cysts on ovaries and other problems may influence hormone levels
• certain medications- chemotherapy, antipsychotic drugs and hormones may affect your periods

Secondary dysmenorrhea is caused by an underlying problem such as:

• IUDs (intrauterine devices)
• uterine abnormalities such as fibroids (abnormal growths in the uterus)
• endometriosis (a condition in which uterine tissue is present outside the uterus)
• PID (pelvic inflammatory disease) caused by infection of the uterus, fallopian tubes and/or ovaries)

When periods are abnormally long (lasting more than a week) or very heavy, with large clots and episodes of flooding, anemia can occur. There are many problems that can cause menorrhagia, such as:

• uterine fibroids or polyps
• a bleeding tendency or disorder
• cancer (rarely)
• infection
• thyroid disease
• kidney or liver disease
• hormonal imbalances
• certain medications (aspirin, anticoagulants)
• IUDs