There are two ways to use this method, the Billings method or the TwoDay method.
• The Billings method is the oldest method of observing cervical secretions to determine the fertile period. This method takes time and practice by learning to observe, record, and interpret cervical secretions, paying close attention to the color, elasticity, amount, and viscosity. Unprotected intercourse should be avoided on menses, on pre-ovulatory days following days with intercourse due to the possible confusion with semen, on all days with wet, slippery, transparent, or stretchy secretions consistent with ovulation, and until four days past the last day with wet secretions. Based on these rules, you need to avoid unprotected intercourse for approximately 14 to 17 days of each cycle. This method can be up to 97% effect with perfect use but more likely around 77% with typical use.
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• The TwoDay method uses the presence or absence of cervical secretions in determining the fertile period. Unprotected intercourse should be avoided on all days that secretions are noted as well as the first day following a day with secretions. This method can be up to 97% effective with perfect use but is around 86% effective with typical use.
Institute for Reproductive Health