"The Billings Method is a unique system of natural family planning that provides women with a simple marker of their fertility, a signal that they themselves can recognise. It is based on the knowledge that changing hormone levels throughout your menstrual cycle affect the volume and texture of mucus that is produced in the cervix and appears at the vaginal opening, and once you are familiar with these changes you can accurately identify the time when you will ovulate" Dr. Evelyn Billings and Dr. Ann Westmore (The Billings Method: Using the body's natural signal of fertility to achieve or avoid pregnancy, 16th ed)