How Period Predictions Work
Each cycle starts on the predicted first day of the period. Ovulation is estimated at cycle length minus 14 days from the start of the period. The fertile window spans 5 days before ovulation through the day of ovulation. The calculator shows the next 6 cycles.
Frequently Asked Questions
The next period date is estimated by adding your average cycle length to the first day of your last period. A typical menstrual cycle ranges from 21 to 35 days, with 28 days being the average. Cycle length can vary by several days from month to month, so treat predictions as estimates.
The fertile window is the 5-6 day period around ovulation when pregnancy is possible. Sperm can survive in the reproductive tract for up to 5 days, and the egg is viable for about 24 hours after ovulation. The fertile window typically starts 5 days before ovulation and ends on the day of ovulation.
Ovulation typically occurs about 14 days before the next period, regardless of cycle length. For a 28-day cycle, this is day 14. For a 35-day cycle, it would be around day 21. For a 21-day cycle, around day 7. This calculator estimates ovulation as (cycle length - 14) days after your last period.
Menstrual cycles can vary due to stress, changes in weight, illness, travel, hormonal changes, thyroid conditions, polycystic ovary syndrome (PCOS), and perimenopause. If your cycle regularly varies by more than 7-9 days, consult a healthcare provider.
Period predictions are estimates based on a consistent cycle length. For people with regular cycles, predictions are often within 1-3 days. For irregular cycles, predictions are less reliable. Tracking your cycle over several months improves accuracy.
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