You'll start to notice signs that you're fertile about five days before you ovulate. Ovulation usually occurs between 12 and 14 days before your period starts (Fehring et al 2006) . This is an average, so it could be a couple of days earlier or later. For example, say you have a regular 28-day menstrual cycle. Count the first day of your period as day one. Your fertile window is likely to be around days 12 to 17. However, a lot of women have an irregular cycle. If your cycle is irregular, ovulation may occur a week earlier or later from one month to the next (Fehring et al 2006) .