Fertility period is determined after accurately recording the lengths of menstrual cycles for at least 6 months. Subtract 18 from the shortest cylce and 11 from the longest cycle to figure fertile days.
- Shortest cycle: 24 - 18 = 6th day
- Longest cycle: 30 - 11 = 19th day
Abstinence of sex for day 6 to day 19.