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The ovulation day is counted backward from the last day of the menstrual cycle.
For example, if a woman had a regular cycle of 30 days so she will ovulate 14 days prior to her last day of that cycle. Meaning, she ovulates on the 16th day of her cycle.
Another example is that if a lady has a regular cycle of 28 days then her ovulation will occur 14 days prior to her last day. Meaning, she will ovulate on the 14th day of her cycle.
It is so because the menses start exactly after 14 days of ovulation.
For more information consult a sexologist online --> https://www.icliniq.com/ask-a-doctor-online/sexologist