Find out when you're going to ovulate and it will increase the chance of you getting pregnant. All you have to do is tell us the first day of your last period and how long your cycle usually lasts. (For some women this can be 20 days and for others 46 days or more) Once you've told us the length of your cycle, we'll tell you when you're most likely to be fertile over the next 3 months. It's important to remember that our Ovulation Calendar only provides approximate dates. If you have an irregular menstrual cycle then your ovulation dates are a bit more difficult to get right.