Your menstrual cycle is far more than your period. It is a roughly month-long hormonal rhythm that prepares your body for a possible pregnancy each month, and it influences your energy, mood, skin, and sleep along the way. Understanding it is the foundation for everything from tracking your period to trying to conceive.
The four phases of the menstrual cycle
A cycle is counted from the first day of your period (day 1) to the day before your next period. Most cycles move through four phases driven by rising and falling levels of estrogen and progesterone.
- Menstrual phase (days 1–5): the uterine lining sheds — your period.
- Follicular phase (days 1–13): estrogen rises, an egg matures, and energy often climbs.
- Ovulation (around day 14): an egg is released; this is your most fertile window.
- Luteal phase (days 15–28): progesterone rises; if there’s no pregnancy, hormones fall and the next period begins.
What is a normal cycle length?
A “textbook” cycle is 28 days, but anywhere from 21 to 35 days is considered normal for adults, and cycles are often longer and more variable in the first few years after your first period and again in perimenopause. What matters most is what is normal *for you* — sudden changes in length, flow, or pain are worth tracking and discussing with a clinician.
How to track your cycle
Tracking even a few markers — period dates, flow, and symptoms — reveals your personal pattern and improves the accuracy of period and fertility predictions. You can start with our free skin type and routine tools for the skin side of your cycle, and use the Eve app to log periods, symptoms, and ovulation signs in one place.
If your cycles are irregular, very heavy, or painful, see our guides on irregular periods and period cramps, and consider speaking to a women’s health provider.
For readers in United States
In the US, the privacy of reproductive-health data is a key consideration — choose apps and providers that are transparent about how your data is stored and shared, and confirm options with your healthcare provider or insurer.