The mature ovum (egg) released at ovulation only remains capable of being fertilized for about 6 to 24 hours. After that it dies. Sperm remain fertile for between 1 and 3 days. Knowledge of this limited time during which fertilization can take place is utilized for natural methods of family planning.

NFP is based on close observation of your body in the course of the cycle, particularly of the changes in body temperature and the consistency of the cervical mucus. The method is therefore also called fertility awareness.

Several methods of self-observation have been developed to allow identification of the fertile and infertile phases of the menstrual cycle. These are the calendar method, the temperature method, the cervical mucus method and the symptothermal method.

If you are using NFP you must abstain from intercourse during the fertile phase of the cycle. The method is therefore also known as periodic abstinence. It is also possible to use a barrier method (e.g. diaphragm, cervical cap, chemical contraceptives, condom) on the fertile days.

All the methods mentioned are relatively time-consuming. Contraceptive computers are designed to make the natural methods somewhat more user-friendly.