NFP concentrates on female fertility because men are, ordinarily, continuously fertile from puberty throughout life. A woman's body provides three basic signs for identifying the fertile and infertile phases of her cycle. Recognizing the pattern of these physical signs forms the basis for all methods of NFP.
The primary sign of fertility is mucus released by the cervix. A woman learns to identify this healthy cervical mucus which indicates the days that intercourse is most likely to result in pregnancy. A second sign is the basal body temperature. Due to hormonal activity, a woman's resting temperature changes in the course of her cycle: lower before ovulation, higher after. The third sign of fertility is the change in the cervix itself. Years of research on these physiological changes have produced specific rules which the NFP user learns to apply to these observations.