Menstrual Cycle Tracker

How cycle tracking works and why it helps

The tracker counts forward from the first day of your last period. Day one is the first day of real bleeding, not spotting. Using your average cycle length, it predicts your next period, and it estimates ovulation as about 14 days before that. The more cycles you log, the better you can see your own pattern rather than relying on the textbook 28-day cycle, which many people do not have.

The phases of the cycle

• Menstrual phase: the uterine lining sheds. Bleeding usually lasts two to seven days.
• Follicular phase: from the first day of bleeding until ovulation, follicles in the ovary develop and estrogen rises, rebuilding the uterine lining. This phase varies most in length.
• Ovulation: a surge of luteinizing hormone triggers release of an egg, usually once per cycle.
• Luteal phase: the ruptured follicle becomes the corpus luteum and produces progesterone. If pregnancy does not occur, hormone levels fall and the next period begins, usually 10 to 16 days after ovulation.

What counts as typical

For adults, ACOG describes cycles of 21 to 35 days as typical, and cycles in the first years after periods begin can range from 21 to 45 days. Some variation month to month is normal. A cycle that is consistently shorter than 21 days or longer than 35, periods that stop for three months or more, or cycle length that swings widely from month to month are worth discussing with a clinician.

What to log beyond dates

Clinicians find it useful when you can describe how heavy your flow is, how many pads or tampons you use on the heaviest day, whether you pass large clots, how painful your periods are, any bleeding between periods, and symptoms such as mood changes, headaches or breast tenderness and when in the cycle they occur. A few months of notes can make an appointment much more productive.

Privacy

Health data stored in apps may be shared with third parties depending on the app's policies. Read the privacy policy of any tracking app you use, and consider a paper log or an app that stores data only on your device if privacy matters to you.

Limits of any period predictor

Prediction works best when cycles are regular. If you have recently stopped hormonal contraception, are breastfeeding, have just given birth, are approaching menopause, or have a condition such as PCOS or a thyroid disorder, your cycles may not follow a predictable pattern and estimates can be off by a week or more. The predicted fertile window should never be used as your only method of preventing pregnancy.

If a period is late and you could be pregnant, take a home pregnancy test. Most are accurate from the first day of a missed period, and a negative result can be repeated a few days later if your period still has not started.

Changes in your usual pattern matter more than any single number. If cycles that were regular become irregular, if bleeding suddenly gets much heavier, or if periods become far more painful than they used to be, mention it to a clinician even if each cycle still falls within the typical range.

Sources: ACOG: Menstruation in girls and adolescents; Office on Women's Health: Your menstrual cycle; Mayo Clinic: Menstrual cycle, what's normal; FTC: Health apps and privacy.