It will, however, occur around 12 to 14 days before the expected date of your next period, which is the fixed part of your cycle as explained above.
Note that if your cycle lengths vary widely, this calculator will not be useful to you.
Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (assuming a 28 day cycle).
Note that your menstrual period and ovulation are counted as the first two weeks of pregnancy.
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