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Yes, this is 100 percent true—if your menstrual cycle happens to be 28 days. But the majority of women have cycles longer or shorter than this or that are irregular. Ovulation occurs approximately 14 days before your next period begins, so if your cycle is 35 days, then you are ovulating on day 21. If your cycle is 22 days, then you are ovulating on day 8. And if your cycle is a different length every month, well then it’s pretty hard to pinpoint exactly when you’ll be ovulating.