A method of birth control which depends on observing and interpreting changes in cervical mucus throughout the menstrual cycle. Around the time of ovulation, the mucus becomes slippery and stretchy like egg whites. This mucus, called fertile mucus, has a structure that facilitates the passage of sperm into the cervical canal and nourishes them. Women using this method abstain from coitus when they detect fertile mucus. Also called fertility detection or fertility awareness.