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Predict next periods, fertile windows, and ovulation dates from your cycle length.
The menstrual cycle is a monthly hormonal cycle that prepares the uterus for potential pregnancy. A typical cycle lasts 21–35 days, with the average being 28 days, divided into menstrual, follicular, ovulatory, and luteal phases.
Cycle 1
Cycle 2
Cycle 3
| Type | Duration | Notes |
|---|---|---|
| Short Cycle | < 21 days | May indicate hormonal issues |
| Normal Cycle | 21–35 days | Typical range |
| Average Cycle | 28 days | Most common length |
| Long Cycle | > 35 days | May indicate PCOS or other conditions |
| Short Period | 2–3 days | Normal for some |
| Average Period | 4–6 days | Most common duration |
| Long Period | > 7 days | Worth discussing with a doctor |
Formula
Next period = last period start + cycle length | Fertile window = ovulation day ± 5 daysCycle length = Days from first day of one period to first day of the next (average: 28 days, range: 21–35)
Ovulation day = Typically cycle length − 14 days from the start of the period
Fertile window = 5 days before ovulation + ovulation day — when conception is possible
Worked Example
Next period for a 28-day cycle starting Jan 1
Did you know? The average menstrual cycle is 28 days, but research on 612,613 cycles (Clue app, 2019) found that only 13% of cycles are exactly 28 days long. The most common cycle length is actually 29 days, and cycle length varies by up to 9 days within the same individual over a year.
Sources
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