Calendar methods assume a stable luteal phase near two weeks—a population average, not a guarantee for you personally. We anchor cycle day 1 to the first day you report as menstrual bleeding, add your average cycle length to find the next period, and place ovulation two weeks before that next start. The fertile window extends backward to include days sperm might survive. For more fitness and lifestyle planning around conception or pregnancy, use this page alongside our pregnancy and due-date tools.

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