The Calendar Rhythm Method was the only natural method known between the 1930’s and 1950’s. With the Calendar Rhythm Method, an attempt was made to establish the infertile phase based on previous cycles. Irregular cycles introduce an element of uncertainty into the calculation. Ovulation and fertility can vary without the couple being aware of it. This can lead to unexpected pregnancy.
The Sympto-Thermal Method (STM) is a modern, effective, and natural method. STM works equally well for women with regular or irregular cycles because it is based on the observation of day-to-day changes of fertility symptoms. The STM focuses on confirming fertile and infertile times within the cycle by charting changes in mucus and basal body temperature, as well as observing other fertility signs.