Calendar or rhythmic method of calculating fertile and infertile days

The calendar-based or rhythmic method (the Knaus-Ogino method) is one of the natural methods of contraception (periodic abstinence) in which the fertile period is estimated according to the length of previous menstrual cycles and the time of ovulation. It has a high failure rate because the length of the next cycle is predicted based on the length of the menstrual cycle from the past. The calendar method calculates fertile – “dangerous days for pregnancy” and infertile – “safe” days between two menstruations. Sexual intercourse should be avoided during fertile days.

How to calculate fertile days?

A woman’s fertile period lasts 5 days before and 3 days after ovulation, which always occurs 14 days before the next menstruation. The time of ovulation is calculated by subtracting 14 days from the number of days of the menstrual cycle. Then, from that calculated day of ovulation, we subtract 5 days and add 3 days from the day of ovulation. Thus, the time of the fertile period or fertile days is obtained. During fertile days, sexual intercourse should be avoided because there is a high possibility of fertilization and conception. 

Calculation examples:

• If the menstrual cycle lasts 28 days, then subtract 14 from that number and get the day of ovulation = 14th day of the cycle (the 14th day from the beginning of menstruation). If ovulation is the 14th day of the cycle, then the fertile days will be from the 9th to the 17th day of the cycle. The other days in that cycle are infertile. 

28 – 14 = 14th day – OVULATION 

14 – 5 = 9th day (beginning of fertile days) 

14 + 3 = 17th day (end of fertile days)

• If the menstrual cycle lasts 30 days, then we subtract 14 days from that number and thus get the ovulation day = 16th day of the cycle (the 16th day from the beginning of menstruation). If ovulation is on the 16th day of the cycle, the fertile days will be from the 11th to the 19th day of the cycle. The other days in that cycle are infertile. 

30 – 14 = Day 16 – OVULATION 

16 – 5 = 11th day (beginning of fertile days) 

16 + 3 = 19th day (end of fertile days)

• If the cycles are unequal, then it is necessary to establish the duration of the shortest and longest cycle for the previous year. Then 18 is subtracted from the shortest cycle and 11 is subtracted from the longest cycle. Thus, a period of fertile days is obtained when one abstains from sexual intercourses.

Example:

If the cycles vary from 25 to 32 days, then 18 days are subtracted from the shortest cycle (25 days) and the 7th day is when fertile days begin. From the longest cycle (32 days), 11 days are subtracted and the 21st day of the cycle is when fertile days end. According to the calculation, sexual intercourse should be avoided during fertile days, which is from the 7th day of the cycle (calculated from the beginning of menstruation) to the 21st day of the cycle.

25 (shortest cycle) – 18 = 7th day (beginning of fertile days) 

32 (longest cycle) – 11 days = 21st day (end of fertile days)