Contraception

Methods of Contraception: Contraceptive Methods for Women: “Rhythm”

Calendar
A second way of finding the "unsafe" days consists in keeping a calendar record of menstrual cycles for at least eight months. Using this calendar record, a woman selects her longest and her shortest cycle. She then subtracts 18 days from her shortest cycle to find the first "unsafe" day of her present cycle. She further subtracts 11 days from her longest cycle to find the last "unsafe" day of her present cycle. For example, if her shortest cycle is 25 days, then 25 - 18 = 7 (the seventh day is the first "unsafe" day of her present cycle): If her longest cycle is 31 days, then 31-11= 20 (the twentieth day is the last "unsafe" day of her present cycle).

Shortest cycle    First unsafe day after beginning of cycle Longest cycle    Last unsafe day after beginning of cycle 21 days -18 =   3rd day 21 days -11 = 10th day 22 days -18 =   4th day 22 days -11 = 11th day 23 days -18 =   5th day 23 days -11 = 12th day 24 days -18 =   6th day 24 days -11 = 13th day 25 days -18 =   7th day 25 days -11 = 14th day 26 days -18 =   8th day 26 days -11 = 15th day 27 days -18 =   9th day 27 days -11 = 16th day 28 days -18 = 10th day 28 days -11 = 17th day 29 days -18 = 11th day 29 days -11 = 18th day 30 days -18 = 12th day 30 days -11 = 19th day 31 days -18 = 13th day 31 days -11 = 20th day 32 days -18 = 14th day 32 days -11 = 21th day 33 days -18 = 15th day 33 days -11 = 22th day 34 days -18 = 16th day 34 days -11 = 23th day 35 days -18 = 17th day 35 days -11 = 24th day 36 days -18 = 18th day 36 days -11 = 25th day etc.       etc.