Fertility Awareness - Cycle History

Cycle History

Calendar-based systems determine both pre-ovulatory and post-ovulatory infertility based on cycle history. When used to avoid pregnancy, these systems have higher perfect-use failure rates than symptoms-based systems, but are still comparable to barrier methods such as diaphragms and cervical caps.

Mucus- and temperature-based methods used to determine post-ovulatory infertility, when used to avoid conception, result in very low perfect-use pregnancy rates. However, mucus and temperature systems have certain limitations in determining pre-ovulatory infertility. A temperature record alone provides no guide to fertility or infertility before ovulation occurs. Determination of pre-ovulatory infertility may be done by observing the absence of fertile cervical mucus; however, this results in a higher failure rate than that seen in the period of post-ovulatory infertility. Relying only on mucus observation also means that unprotected sexual intercourse is not allowed during menstruation, since any mucus would be obscured.

Use of certain calendar rules to determine the length of the pre-ovulatory infertile phase allows unprotected intercourse during the first few days of the menstrual cycle, while maintaining a very low risk of pregnancy. With mucus-only methods, there is a possibility of incorrectly identifying mid-cycle or anovulatory bleeding as menstruation. Keeping a BBT chart enables accurate identification of menstruation, when pre-ovulatory calendar rules may be reliably applied. In temperature-only systems, a calendar rule may be relied on alone to determine pre-ovulatory infertility. In symptothermal systems, the calendar rule is cross-checked by mucus records: observation of fertile cervical mucus overrides any calendar-determined infertility.

Calendar rules may set a standard number of days, specifying that (depending on a woman's past cycle lengths) the first three to six days of each menstrual cycle are considered infertile. Or, a calendar rule may require calculation, for example holding that the length of the pre-ovulatory infertile phase is equal to the length of a woman's shortest cycle minus twenty-one days. Rather than being tied to cycle length, a calendar rule may be determined from the cycle day on which a woman observes a thermal shift. One system has the length of the pre-ovulatory infertile phase equal to a woman's earliest historical day of temperature rise minus seven days.