Where the 28-Day Myth Came From
The 28-day cycle became medical orthodoxy partly because it aligned conveniently with the lunar month and partly because early menstrual research was conducted on small, homogenous groups of women. A 2019 study of 600,000 menstrual cycles found that only 13% lasted exactly 28 days, and cycle length varied significantly from month to month even within the same individual.
What "Normal" Actually Looks Like
A healthy cycle can range from 21 to 35 days. Periods lasting 3–7 days and blood loss of 30–80ml are within normal range. Variation of 7–9 days between your shortest and longest cycle is common. A cycle that's consistent for you, even if it's 35 days, is healthier than an "average" 28-day cycle that varies wildly.
When Variation Signals Something Worth Investigating
Significant cycle changes, especially sudden lengthening, shortening, increased heaviness, or new pain, are worth discussing with your doctor. PCOS often presents with very long or absent cycles. Thyroid disorders can shorten or lengthen cycles. Perimenopause causes increasing irregularity. The key is change from your normal, not deviation from an arbitrary average.
Medical Disclaimer
This article is written for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making health decisions.
Dr. Emma Walsh
FRCOG
All TryHerCare articles are written and reviewed by qualified medical professionals. Our content is clinician-reviewed to ensure accuracy and clinical relevance.
