Dr. Anushree Pande | Gynaecologist on Instagram: "Here’s how to calculate your fertile window ⬇️ Use the calendar method to track the length of your menstrual cycle. 🩸 Each month, mark the first day of your period on a calendar or in a period-tracking app. 🩸The number of days between the first day of consecutive periods is the length of your menstrual cycle. You should do this for at least six months to get good data. 🩸You ovulate about 12 to 14 days before the start of a new menstrual cycle.