During your cycle, the type and amount of cervical mucus — a secretion made by glands in your cervix — changes. It can be sticky or stretchy, white or cloudy. By making a note of these changes, you can predict when you will ovulate so you can have sex on your most fertile days.
The main advantage of this method is that it gives you a better understanding of your body, making you more aware of the changes your body goes through during a cycle. However, it can take time to learn how to ‘read’ the changes, and your cervical mucus can be influenced by other factors, including infections, sex and some medication.