Ovulation is very tricky to pinpoint. Every person is different, and ovulation can be affected by external factors like stress, diet or medical conditions like PCOS (Polycystic Ovary Syndrome). Even if your menstrual cycles seem regular, the ovulation date is unpredictable and could come at any time during your cycle – as well as sometimes changing between cycles. Long story short: you may be fertile at almost any point during your menstrual cycle (yes, even when you are on your period) and unprotected sex at any time of the month is not a good idea when trying to avoid an unplanned pregnancy.