What causes irregular periods?

Bottom lineIrregular periods are commonly caused by puberty or perimenopause, PCOS/PMOS, thyroid problems, stress, big weight or exercise changes, and starting or stopping hormonal birth control; see a doctor for cycles consistently under 21 or over 35 days, three or more missed periods when not pregnant, very heavy or prolonged bleeding, or irregularity with symptoms like acne or excess hair.

Irregular periods mean the timing, length, or flow varies a lot from cycle to cycle. Occasional irregularity is common; a persistent pattern is worth investigating.

Common causes

• Puberty and perimenopause - cycles are naturally irregular at both ends of reproductive life
• PCOS/PMOS - one of the most common causes of irregular or missed periods
• Thyroid problems - both over- and underactive thyroid
• Stress and poor sleep
• Significant weight change or intense exercise
• Hormonal birth control - starting, stopping, or switching
• High prolactin or other hormonal imbalances
• Perimenopause

When to see a doctor

• Cycles consistently shorter than 21 or longer than 35 days
• Missed periods (3 or more in a row) when not pregnant
• Very heavy or prolonged bleeding
• Irregularity plus other symptoms (acne, excess hair, weight changes) that may point to PCOS/PMOS

What helps

Identifying the cause is the key. Tracking your cycles makes the pattern visible and gives your clinician something concrete to work with.

Learn the basics in what is PCOS, and map your cycles with the Menstrual Cycle Calculator.

Femora reveals irregular patterns over time so you can act on them earlier.