A variety of indicators may be used to establish the period most safe for intercourse. These include:
calendar or rhythm method - using a menstrual calendar to calculate the time of ovulation. Assuming a regular 28 day cycle with ovulation on the 14th day, intercourse should be avoided from 4 days before until 3 days after ovulation. If the cycle length is variable, the "unsafe" period is determined by subtracting 18 days from the shortest cycle and 11 days from the longest cycle. For example, if shortest cycle is 26 days, and longest is 32 days, the "unsafe" period is day 8 to day 21.
temperature - basal body temperature rises by 0.5 degree celcius in the last 14 days of a 28 day cycle following ovulation. Intercourse should be avoided from the 5th day after the menses until at least 3 days after ovulation as indicated by the change in BBT. Fever and anti-pyretics may both make BBT unreliable.
cervical mucus change - the mucus is profuse, thin and clear during the ovulatory phase of the cycle but scant, thick, and opaque pre- and post-ovulation. Intercourse should be avoided from the appearance of ovulatory mucus until after 2 - 3 days of post-ovulatory mucus.
symptoms - e.g. mid-cycle pain, bleeding, headache. Helpful in addition to other natural methods.
LH detection kits - detect the LH surge in urine 36 hours before ovulation.