In the first couple of years after periods begin and in the years approaching menopause, irregular cycles are expected as hormones fluctuate. Outside these phases, common causes include polycystic ovary syndrome, thyroid disorders, significant stress, large weight changes, excessive exercise, and certain medicines, including some hormonal contraceptives.
Less commonly, irregular periods can relate to other hormonal issues, such as raised prolactin, or to structural problems in the uterus. Pregnancy should also be considered when periods change unexpectedly.
See a doctor if your periods are persistently irregular, if you go several months without one, if cycles are very short or very long, or if irregularity comes with symptoms like unusual hair growth, acne, milky nipple discharge, or difficulty conceiving. Tracking your cycles helps the clinician spot the pattern and decide on tests.