• The fertility indicator(s) used,
• The validity of the rules used,
• The type and quality of teaching,
• The ability of the woman to observe and interpret her fertility signs,
• The ability of the couple to abstain from vaginal intercourse when indicated by the rules, and
• Individual characteristics of the couple, such as the age of the woman and her history of pelvic infection or other determinants of lowered fertility.

Rationale: It is difficult to estimate the effective- ness of the calendar method because very few well-designed studies have been conducted. Many of the estimates are based upon surveys and do not report the requisite information required to calculate effectiveness rates. It is difficult to compare studies on the calendar method because many reports do not state what rule was used to identify the fertile time or if couples understood the basis for the method (65, 165).

Rationale: The probability of accidental pregnancy when using single indicator methods is highest when couples do not abstain during the fertile period. For users of the cervical mucus or ovulation methods, having intercourse during periods of stress on the woman also increases the risk of pregnancy by affecting the quality of mucus. For users of the BBT method, stress, illness, travel, or interrupted sleep can disrupt a woman's typical biphasic pattern, thereby making it difficult to identify the fertile period for that cycle using the body temperature (114, 275, 356).

Rationale: The use of two or more indicators can be somewhat more effective than the use of a single indicator (90, 114).

Rationale: Most women who are regularly cycling can use NFP. Women who are postmenarche, postpartum, breastfeeding, postabortion, or perimenopausal can also use NFP. However, since the hormonal patterns are altered, a woman may find it more difficult to interpret her fertility signs and must follow special rules to track her fertility. The specific rules used will depend upon the circumstances and the NFP method chosen. Calendar-based methods are not recommended for women in the situations mentioned above because many of these cycles are not ovulatory and are often very irregular, requiring prolonged required abstinence intervals, which may be difficult for many couples (307).

Recommendation: Because of the large range of failure rates of NFP methods, women with conditions that may be seriously affected by pregnancy must be counseled on the degree of risk of an unintended pregnancy.

Rationale: The higher range of failure rates of these methods may expose the user to an unacceptable risk of unintended pregnancy (302). (See Question 1.)

It is not recommended that women rely on the cervical mucus method if they have an abnormal vaginal discharge.

Rationale: Abnormal pathologic discharge interferes with a woman's ability to observe changes in cervical mucus patterns. If a woman has an abnormal discharge, she should be referred to a health care provider for appropriate diagnosis and treatment. If possible, couples should abstain until treatment is complete or be advised to use condoms to prevent re-infection. The male partner should also be treated at the same time, if required. Once the discharge has returned to normal, women can begin using the cervical mucus method.

Rationale: NFP is a "couple method" and requires good communication and cooperation between partners. NFP is not only a method of family planning, but it is also a method of self-knowledge and self-control and mutual respect.

Recommendation: NFP requires the ability to observe and interpret fertility indicators accurately.

Rationale: Couples must be willing to monitor the changes in the woman's fertility signs consistently so that the fertile time can be identified accurately.

Recommendation: NFP requires abstinence from vaginal intercourse for up to half the days of the menstrual cycle.

Rationale: On average, most NFP methods require 10 to 12 days of abstinence per cycle. The length of abstinence depends upon the NFP method chosen and individual characteristics of the woman's cycle.

Recommendation: If the man or the woman abuses drugs or alcohol, offer another family planning method.

Rationale: If either the man or the woman abuses alcohol or drugs, it will be difficult for the couple to follow the rules of abstinence.

Recommendation: If the male partner is not interested in NFP, then the woman or couple should be informed of alternative methods.

Rationale: NFP will not be effective in preventing unplanned pregnancy if the male partner is unwilling to accept abstinence during the fertile period. Such couples should be informed of other family planning options.

Recommendation: If the man or the woman is at risk for acquiring or transmitting a STD, the couple should be counseled about risk factors for STD transmission.

Rationale: NFP is not protective against STDs, and couples at risk should be informed that to reduce their risk, they should abstain or use condoms.

Rationale: Women with irregular menstrual cycles can still ovulate and be fertile. The fertile period in such women can only be identified by prospective NFP methods such as cervical mucus and BBT. In the presence of irregular cycles, the period of abstinence may be excessively long and unacceptable to some couples.

Rationale: In the absence of relatively regular cycles, it is impossible for a woman to predict the fertile days in her current cycle using the calendar method. As cycle length variation increases, the number of days of abstinence increases. The most common "rule" used to predict the fertile time using a formula is subtracting a certain number of days from the length of the shortest (usually 18 to 20) and longest cycles (usually 8 to 10) in the past 6 to 12 months. If, for example, the longest cycle is 45 days and the shortest cycle is 20 days, a couple would have to abstain for more than 30 days in the woman's current cycle.

Cycle length is determined by starting to count on Day 1 of menses and counting forward until the day before the next menses begins.

Prolonged abstinence may lead to increased risk taking and greater risk of pregnancy if intercourse occurs during the fertile time (166).

Rationale: When the fertile time is predicted using a "blanket" rule—in which a set number of days each cycle is identified as the fertile time—the length of abstinence is not increased if the woman has irregular cycles. An example of a "blanket" rule is if all couples abstain starting on Day 8 of the woman's cycle and resume vaginal intercourse on Day 21. Couples using this rule would abstain for 13 days each cycle, regardless of whether the woman has irregular cycles. However, in the presence of long and irregular cycles, it is very likely that the couple will have intercourse on a fertile day since ovulation usually occurs around 2 weeks before the next menstruation. Thus a woman with a 39-day cycle would resume intercourse on day 21 and probably ovulate around day 25 (166).

Rationale: No drugs or devices are used with NFP. Fertility-awareness-based methods produce no medical side effects. However, periodic abstinence and fear of unplanned pregnancy may create tension and psychological stress in some couples (114).

Recommendation: The best evidence indicates that there is no increased risk to the fetus associated with the use of NFP.

Rationale: Recent studies show that there is no increased risk of spontaneous abortion, small birth size, low birth weight, or malformation to fetuses among users of NFP (31, 103).

For example, women who wish to breastfeed can use the lactational amenorrhea method (LAM) in lieu of, or before, using NFP. When the criteria for LAM use are no longer met (fully or nearly fully breastfeeding, infant less than 6 months old, no menses return), the woman can use NFP to identify her potentially fertile days.