It has long been known that women are fertile for only a few days of the menstrual cycle. The periodic abstinence or rhythm method of contraception requires that coitus be avoided during the time of the cycle when a fertilizable ovum and motile sperm could meet in the oviduct. Fertilization takes place within the tube, and the ovum remains in the tube for about 1-3 days after ovulation; hence the fertile period is from the time of ovulation to 2-3 days thereafter.

Accurate prediction or indication of ovulation is essential to the success of the rhythm method. Data from surveys in developed and developing countries done during the past decade indicate the use of natural family planning methods varies from 0-11%. Pregnancy rates vary, but most reliable studies report 1-year life table pregnancy rates between 10 and 25 per 100 woman years. Accordingly, the types of periodic abstinence vary in their approaches to determining the fertile period.

(1) The calendar method predicts the day of ovulation by means of a formula based on the menstrual pattern recorded over a period of several months. Ovulation ordinarily occurs 14 days before the first day of the next menstrual period. The fertile interval should be assumed to extend from at least 2 days before ovulation to no less than 2 days after ovulation. An overlap of 1-2 days of abstinence either way increases the likelihood of success. Successful use of this approach is based on the knowledge that the luteal phase of a menstrual cycle is relatively constant at 14 days for normal women. Furthermore, for this approach to be successful as the only form of contraception requires regular menstrual cycles so that the various timing schedules retain validity. Although this is the most commonly used method of periodic abstinence, it is also the least reliable with failure rates as high as 35% in 1 year’s use.

(2) A somewhat more efficacious approach to periodic abstinence is the temperature method, since more reliable evidence of ovulation may be obtained by recording the basal body temperature (BBT). The vaginal or rectal temperature must be recorded upon awakening in the morning before any physical activity is undertaken. Although it is often missed, there is a slight drop in temperature 24-36 hours after ovulation. The temperature then rises abruptly about 0.3-0.4 °C (0.5-0.7 °F) and remains at this plateau for the remainder of the cycle. The third day after the onset of elevated temperature is considered to be the end of the fertile period. For reliability, care must be taken by the woman to ensure that true basal temperatures are recorded, ie, that hyperthermia due to other causes does not provide misleading information. A distinct limitation of this technique is that prediction of timing of ovulation in any given cycle is retrospective, making it difficult to predict the onset of the fertile period.

(3) The combined temperature and calendar method uses features of both the previously mentioned methods to more accurately predict the time of ovulation. Failure rates of only 5 pregnancies per 100 couples per year have been reported in studies of well-motivated couples.

(4) The cervical mucus (Billings) method uses changes in cervical mucus secretions as affected by menstrual cycle hormonal alterations to predict ovulation. Starting several days before and until just after ovulation, the mucus becomes thin and watery, whereas at other times the mucus is thick and opaque. Women using this approach are trained to evaluate their mucus on a daily basis. Success rates are similar to those described for the combined temperature and calendar method. Advantages of this approach include relative simplicity and lack of a requirement for charting; disadvantages include difficulty in evaluating mucus in the presence of vaginal infection and the reluctance of some women to evaluate such secretions.

(5) The symptothermal method, if used properly, is probably the most effective of all the periodic abstinence approaches. It combines features of both the cervical mucus and the temperature methods. In addition, symptoms that may occur just prior to ovulation such as bloating and vulvar swelling are used as adjuncts to predict the likely occurrence of ovulation.

The most accurate method of determining ovulation time is to demonstrate the luteinizing hormone (LH) peak in serum specimens. Because of the cost and the time required for the serial measurements of LH that are essential to indicate the abrupt rise, this method is impractical as a method of birth control. It is valuable in the treatment of infertility, however, when the optimal time for coitus or artificial insemination is of great importance.

Figure 33-1 shows the relationships among ovulation, BBT, serum levels of LH and follicle-stimulating hormone (FSH), and menses. At least 20% of fertile women have enough variation in their cycles that reliable prediction of the fertile period is impossible.

Epidemiologic studies of women using the rhythm method have suggested an increased incidence of congenital anomalies such as anencephaly and Down’s syndrome among children resulting from unplanned pregnancies. Delayed fertilization has been shown in animal experiments to result in an increased incidence of aneuploidy and polyploidy in offspring, thus suggesting a possible explanation for similar human fetal anomalies. However, despite a theoretical explanation for the occurrence of such birth defects, it is important to recognize that much of the data are subject to bias, and it would be inappropriate to conclude that such associations have been conclusively proved.