Natural family planning
Natural family planning (NFP) is a term referring to the family planning methods approved by the Roman Catholic Church.  In accordance with the requirements for sexual behavior maintained by this church, NFP excludes the use of contraception, as well as all orgasmic acts outside of those ending in unprotected vaginal intercourse with the user's spouse.
Periodic abstinence and the natural infertility caused by breastfeeding are the only methods permitted for avoiding pregnancy. Sex during infertile periods is permitted, whether during infertile periods of the menstrual cycle, during pregnancy, or post-menopause.  Several methods may be used to identify whether a woman is likely to be fertile, which NFP users make use of to either try to avoid or to achieve pregnancy.

# Prevalence
It is estimated that 2-3% of the world's reproductive age population relies on periodic abstinence to avoid pregnancy.  Breastfeeding is believed to reduce the world's fertility rate by 30-45%. However, what portion of this population should be considered NFP users is unclear. Some Catholic sources consider couples that violate the religious restrictions associated with natural family planning to not be NFP users.
There is little data on the worldwide use of natural family planning.  In Brazil, NFP is the third most popular family planning method.  The "safe period" method of fertility awareness is the most common family planning method used in India, although condoms are used by some.
Use of NFP in developed countries is low, even among Catholics. While Catholics made up 24% of the U.S. population in 2002, of reproductive age American women using birth control, only 1.5% were using periodic abstinence.
Use of NFP is not restricted to Catholic couples.  In 2002, Sam and Bethany Torode, then a Protestant Christian couple, published a book advocating NFP use.  (Five years after authoring the book, the Torodes redacted their advocacy of NFP-only to also support barrier methods as moral.  The Torodes converted to Orthodox Christianity.)

# Methods
There are three main types of NFP.  These are the observational methods, the statistical or "calendar" methods, and the lactational amenorrhea method.  Observational methods rely on biological signs of fertility, while statistical methods approximate the likelihood of fertility, based on the length of past menstrual cycles.

## Observational
Some methods of NFP track biological signs of fertility.  When used outside of the Catholic concept of NFP, these methods are often referred to simply as fertility awareness methods rather than NFP.  The three primary signs of a woman's fertility are her basal body temperature, her cervical mucus, and her cervical position. Some systems use only cervical mucus to determine fertility. Two well-known mucus-only methods are the Billings ovulation method and the Creighton Model FertilityCare System.  If two or more signs are tracked, the method is referred to as a symptothermal method.  Two popular symptothermal systems are that taught by the Couple to Couple League, and the Fertility Awareness Method (FAM) taught by Toni Weschler. Some fertility monitoring devices accept and interpret temperature readings, use urinalysis to follow changes in levels of estrogen and luteinizing hormone, or both.

## Statistical
These methods make statistical estimates as to when a woman is fertile.  They include the Rhythm Method and the Standard Days Method.

## Lactational amenorrhea
The lactational amenorrhea method (LAM) is a method of avoiding pregnancies which is based on the natural postpartum infertility that occurs when a woman is amenorrheic and fully breastfeeding.  The rules of the method help a women identify and possibly lengthen her infertile period.  A strict version of LAM is known as ecological breastfeeding.

## Effectiveness
The Rhythm Method, which is sometimes considered NFP, but also not classified as NFP by the United States Conference of Catholic Bishops, has a perfect-use failure rate of 9% per year, while observational and lactational methods of NFP have lower perfect-use failure rates  between 1%-3% per year. The common usage failure rate is up to 25% per year.

# Theology

## Official Catholic
Catholic doctrine holds that God created sexual intercourse to be both unitive and procreative.
This church considers deliberately altering fertility or the marital act with the intention of preventing procreation to be sinful. Thus, artificial birth control methods and orgasmic acts outside of full marital intercourse are forbidden, while not having sex at all (abstinence) is considered morally acceptable.  While deliberate orgasmic acts apart from a culmination in full marital intercourse are forbidden, this church does not condemn foreplay in the context of sexual relations within marriage.  Foreplay is unobjectionable as long as the act culminates with the husband penetrating his wife's vagina with his penis and ejaculating while penetrating.  The Catholic church does not condemn accidental premature ejaculation.
Having sex at an infertile time in a woman's life (such as pregnancy or post-menopause) is also considered acceptable, since the infertile condition is considered to be created by God, rather than as an act by the couple.  Similarly, under Catholic theology, it may be morally acceptable to abstain during the fertile part of the woman's menstrual cycle.
Increasing the postpartum infertile period through particular breastfeeding practices  the lactational amenorrhea method  is also considered a natural and morally unobjectionable way to space a family's children.
The Catholic church acknowledges a potential benefit of spacing children
and use of NFP for this reason is encouraged. Humanae Vitae cites "physical, economic, psychological and social conditions" as possibly compelling reasons to avoid pregnancy.
Couples are warned, however, against using NFP for selfish, immoral, or insincere reasons.  A few Catholic theologians argue that couples with several children may morally choose to avoid pregnancy even if their circumstances (emotional, physical, and economic) would allow for more children.
More commonly, Catholic sources extol the benefits children bring to their parents, their siblings, and society in general, and encourage couples to have as many children as their circumstances make practical.

## Dissident and other opposing
Many Catholics have voiced significant disagreement with this church's stance on contraception. The Canadian Conference of Catholic Bishops issued probably the most heavily dissenting document, the Winnipeg Statement. In it, the bishops argued that many Catholics found it very difficult, if not sometimes impossible, to obey Humanae Vitae. Additionally, they reasserted the Catholic principle of primacy of conscience. Catholics such as Fr. Charles Curran have also criticized the stance of Vitae on artificial birth control.
Theological opposition has additionally come from Protestant Christianity.  John Piper's Desiring God ministry states of NFP, "There is no reason to conclude that natural family planning is appropriate but that "artificial" means are not." Sam and Bethany Torode, former advocates of NFP-only, have redacted their position to include barrier methods and explain their current theology this way:

# Potential advantages
Fertility awareness methods of NFP can be used to track reproductive health in general, and aid in the diagnosis of gynecological health and/or fertility problems. Particularly with the Sympto-Thermal Method, which is based on a scientific method of self-observation, temperature, external and internal mucus, and cervical observations are all recorded daily on a calendar. Any observation of irregularities may prove helpful, providing a doctor or gynecolotist with important diagnostic information with dates and a personal history of any abnormalities.  The fertility knowledge gained from NFP methods may also be used to help achieve pregnancy.
NFP has none of the side effects associated with hormonal methods and spermicides, nor the need for specific action in preparation for each act of intercourse required by barrier methods. Prominent theologians, including Pope John Paul II, as well as many NFP users, feel that refraining from barrier methods allows for more spontaneous intercourse during the infertile period.
NFP can be free or very low-cost. Some methods encourage the use of a thermometer and/or photocopied charts, which can be obtained relatively inexpensively. Some couples choose to use software or to use other devices such as a string of beads for counting the days.

## Divorce rate
Proponents of NFP say it increases marital satisfaction and helps lower divorce rates.   Anecdotal evidence and small studies are put forth in support of this view.
One study involving 505 women using NFP found a correlation between lower divorce rates and the use of NFP.

## Claims regarding communication
Fertility awareness forms of NFP (which require daily charting of fertility signs) can result in couples communicating more about their bodies and sexuality; this is said to improve communication skills throughout the entire marriage. One idea, called a "courtship and honeymoon effect", is that the scheduled abstinence encourages couples to express love in non-sexual ways, and to have a greater appreciation for intercourse when it does happen.

# Potential disadvantages

## Abstinence requirement
- The fertility awareness and statistical forms of NFP require periodic abstinence, known as continence, most commonly 8-10 days of each menstrual cycle.  It is common for the woman's sexual desire to be highest on those fertile days, and low in other parts of the cycle. That the couple must abstain from sex during the woman's periods of peak sexual desire may be deeply frustrating for both partners, and lead to feelings of isolation.
- Some methods of NFP require up to six months of research and observation (and abstinence) in order to collect baseline data, so these methods cannot be immediately implemented by a new user. 
- Couples seeking the lowest risk of pregnancy (less than 1% per year) may be required to abstain for more than half of each menstrual cycle.
- For women in certain situations - breastfeeding, perimenopause, or with hormonal diseases such as PCOS - abstinence may be required for months at a time.
- When taught in the context of Catholicism, this church teaches that orgasmic acts outside of intercourse, such as consummated masturbation and oral sex, are morally incompatible with the correct practice of NFP. Some couples are not comfortable with this restriction. Periodic abstinence also limits spontaneous sex.

## Method limitations
- Some fertility awareness forms of NFP require observation or touching of cervical mucus, an activity with which some women are not comfortable. Some practitioners prefer to use the term "cervical fluid" to refer to cervical mucus, in an attempt to make the subject more palatable to these women. Some drugs, such as decongestants, can change cervical mucus.  Women may not be able to rely on these forms of NFP while taking these drugs.
- Some fertility awareness forms require tracking of basal body temperatures.  Because irregular sleep can interfere with the accuracy of basal body temperatures, shift workers and those with very young children, for example, might not be able to rely on those methods.
- The NFP methods that encourage breastfeeding infertility can only be used until the woman's first post-partum menstruation. Even the most strict form results in average return of menses at 14 months post-partum, and menstruation can return as early as 6 weeks post-partum.
- NFP does not protect against sexually transmitted disease.

## Implantation failure and miscarriage
- It has been speculated by some that unprotected intercourse in the infertile periods of the menstrual cycle may still result in conceptions, but create embryos incapable of implanting.
It has also been suggested that pregnancies resulting from method failures of NFP are at increased risk of miscarriage and birth defects due to aged gametes at the time of conception.
The most recent research, however, suggests timing of conception has no effect on miscarriage rates, low birth weight, or preterm delivery.