Understanding your calculation
Method, example and limitations
How the calculation works
A period calendar projects future start dates from the last start date and average cycle length. Cycle length runs from one period start to the next.
Example to check
With a 28-day cycle, adding 28 days to the last period start gives the next estimated start date, rather than adding one calendar month.
Before using the result
Irregular cycles, pregnancy, medication and health conditions can change dates. Predictions do not confirm pregnancy or explain missed or unusual bleeding.