In order to increase the chance of permanency, it is very important to calculate the fertile days, or ovulation days. Here is a very simple and straight forward method to help you calculate your fertile days:
Most women’s period is 28 days. Ovulation usually happens about 14 days after your period, but if your cycle is longer or shorter than 28 days your fertile period will be later or earlier accordingly.
Here is a video to help you to understand more: