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How Period Predictions Work
Period predictions are based on the average length of your menstrual cycle. By adding your cycle length to the first day of your last period, the calculator estimates when your next period will start. Predictions for subsequent cycles repeat this pattern. Ovulation is estimated to occur 14 days before the start of the next period.
Understanding Your Fertile Window
The fertile window spans approximately 6 days each cycle — the 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days in the reproductive tract, so intercourse during this window has the highest chance of conception. Peak fertility is 1-2 days before ovulation.
Frequently Asked Questions
Predictions are most accurate for women with regular cycles. If your cycle varies by more than a few days from month to month, predictions become less reliable. Stress, illness, weight changes, and hormonal factors can all affect cycle timing.
Normal menstrual cycles range from 21 to 35 days, with 28 days being the average. Cycles are counted from the first day of one period to the first day of the next. Consistent cycles within this range are considered normal, even if they are not exactly 28 days.
Calendar-based fertility awareness methods are not highly reliable for birth control (failure rates of 12-24% with typical use). Cycles can vary, and ovulation timing is not perfectly predictable. If avoiding pregnancy, use this tool only alongside more reliable contraception methods.