Natural family planning is the safest method of contraception available, outside of chastity. By safest I refer to no drug-induced side effects or anaphylactic or allergic reactions to chemicals or materials. NFP uses two physiological principles to achieve pregnancy (yes that is it's primary use) or avoid pregnancy. The principles are that of the female's measuring the cyclical thermal shifts and the cyclical cervical mucous viscosity shifts, based on cycles that are no shorter than 26 days. Those women who do have unusually short or long cycles can consult experts to help read the signs properly.

Taking the first day of menstruation as day one of the Cycle, coitus can occur until Cycle day 7 or until the more-fertile mucous is evident. Because the heavy menstrual flow is hostile to sperm life and migration, it is very unlikely that a pregnancy will occur during this time. It has been recorded that a pregnancy did occur after coitus on Cycle day 3, but this was a single case in 40 years of study.Reference unknown, taken as fact from my developmental anatomy professor.

After Cycle day 7, Phase 2 (peak fertility stage) is fast approaching. This is evident by a thermal down-shift and highly viscous mucous. (I am not going to go into details on how to detect this, although checking underwear and/or actually taking some from the cervix will give you this information).

After Phase 3 begins which is 4 days after the peak day (generally Cycle day 14) and cross-checked by 3 full days of a thermal up-shift, coitus can recommence. The importance of cross checking can not be stated enough. The cross checking should also include mucous analysis and result in a thicker less elastic mucous.

To achieve pregnancy, marital relations should occur every other day up through peak day plus 2 and will be evident by a thermal down-shift of 21 days.

Natural Family Planning is not objectively contraception

According to the Roman Catholic Church, if and when a married couple have intercourse at a knowingly infertile time, they are not objectively contracepting for the simple reason that there is nothing to contracept. Note that I said "objectively." For, even if a married couple does not objectively contracept in an infertile time, it is possible that their motivation or intention could be selfish, and this would make their action subjectively contraceptive.

The important point in all of this is that married couples need to nurture a spirituality of generosity, which is actually promoted by the forms of natural family planning that aim primarily at two things:
  • 1) assisting husband and wife in coming to cooperate with God in procreation as knowingly and as willingly as possible
  • 2) leading husband and wife to a reverence for the gift of fertility as the extension in time and space of the activity of the Holy Spirit, the "Lord and Giver of Life."
NFP that promotes these two aims leads to the discovery and nurturing of a uniquely marital experience of the Gospel of Life, of being Temples of the Holy Spirit, and of a spirituality of generosity.

The Roman Catholic Church thus teaches catholic couples to use NFP in planning the right time for children, never avoiding them outright, and always trusting in God as to when the gift of life is appropriate for them.

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