Introduction
Concealed ovulation is a unique human characteristic that limits a couple’s ability to identify the day of the menstrual cycle when ovulation occurs [1]. Because conception requires coitus during the 5 days before or on the day of ovulation, the identification of this 6-day fertile period can allow couples to time coitus to either achieve or avoid pregnancy [2]. For couples wanting to avoid pregnancy, methods of identifying this fertile period have been developed so they can abstain from unprotected coitus on these days [3-5]. These are fertility awareness-based methods of family planning, a name that accurately conveys their conceptual foundation. Other names commonly used for these methods include periodic abstinence, the rhythm method, and natural family planning.
Knowledge of reproductive physiology allows awareness of detectable aspects of the menstrual cycle that can improve the ability to predict the day of ovulation and estimate the fertile period. The most important knowledge includes the understanding of the length and variability of the typical menstrual cycle, the awareness of the time of ovulation during the menstrual cycle, the understanding of cervical mucus changes during the menstrual cycle, the recognition that changes in body temperature are associated with the rise in progesterone levels after ovulation, and the understanding of the effect of coital timing relative to ovulation on the probability of conception. Through fertility awareness-based methods of family planning, couples can use their understanding of these phenomena to prevent pregnancy by limiting unprotected coitus to predicted nonfertile days.
Fertility awareness-based methods of family planning can be categorized as either calendar-based methods, for which effectiveness depends on the regularity of a woman’s menstrual cycle, or physiology-based methods, for which effectiveness depends on a couple’s ability to recognize physiological fertility signs. These methods are generally estimated to have a 25% failure rate during the first year of typical use, which reflects the difficulty of avoiding unprotected coitus during the relatively long fertile period of each menstrual cycle [6]. This high failure rate is similar to that of coitus interruptus, but much higher than those of condoms, hormonal contraceptives or intrauterine devices. Fertility awareness-based methods are most appropriate for couples who are either unwilling or unable to use these other more effective contraceptive methods because of ethical concerns, medical concerns or poverty [7,8].
Couples using fertility awareness-based family planning methods should also be counseled on the use of immediately active contraceptive methods, such spermicides, coitus interruptus and barrier contraceptives, during fertile days, and the availability of emergency contraceptives for recognized contraceptive method failure.
Calendar-based methods of family planning depend upon the regularity of a woman’s menstrual cycle because they predict the timing of the fertile period based on the assumption that the upcoming menstrual cycle will be similar to past menstrual cycles. If a woman is not experiencing regular menstrual cycles, then calendar-based methods should be delayed until regular monthly menses resume. Calendar-based methods of family planning include the calendar rhythm method, which uses arithmetic calculations to predict the fertile period, and the Standard Days method, which eliminates the need for these calculations by limiting use of the method to women with regular 26 to 32 day menstrual cycles.
The biologic rationale for the calendar rhythm method is based on the following assumptions.
- Sperm are not capable of fertilizing an ovum more than 72 hours after coitus.
- An ovum is not capable of being fertilized more than 24 hours after ovulation.
- Ovulation occurs from 12 to 16 days before the onset of the next menses.
A woman using the calendar rhythm method records the length of her menstrual cycles for 6–12 months, noting the length of her longest and shortest menstrual cycles. She then calculates the beginning of her next fertile period by subtracting 18 days from the length of her shortest menstrual cycle. These 18 days account for early ovulation at 16 days before the onset of menses plus 2 days for the time that the sperm are assumed to be potent after coitus. She calculates the end of her fertile period by subtracting 11 days from the length of her longest menstrual cycle [3]. These 11 days account for late ovulation at 12 days before the onset of menses minus 1 day for the time that the ovum is assumed to be fertilizable after ovulation. For example, if the menstrual cycle length varies from 26 to 32 days, the fertile period begins on day 8 and ends on day 21, which is 14 total days. During the upcoming menstrual cycle, the couple should avoid unprotected coitus on these days. The calculations are updated monthly, always using data from the six most recent cycles. The first-year failure rate of this method with perfect use is estimated to be 9% [6].
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