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Next period start = last period start + cycle length (average 28 days, range 21–35). Period duration typically 3–7 days. Fertile window centers around day 14 of a 28-day cycle. Cycle irregularity of ±2 days is normal; larger variation may warrant medical evaluation. Apps that track multiple months build more accurate personalized predictions than fixed-interval estimates.
Next period ~1/29/2026