The first day of the fertile period in the rhythm method of contraception is calculated by -

Correct option is C

Sol. The rhythm method, also known as the calendar method, is a natural family planning technique that involves tracking the menstrual cycle to determine fertile days. To estimate the first day of the fertile window, subtract 18 days from the length of the shortest menstrual cycle. For example, if the shortest cycle is 26 days, then 26 - 18 = 8; thus, the fertile period begins on the 8th day of the cycle. This calculation helps in identifying the days when unprotected intercourse is most likely to result in pregnancy.
Explanation of Each Option:
· Longest cycle -18 days: Incorrect; this calculation does not accurately determine the start of the fertile period.
· Longest cycle -10 days: Incorrect; this formula is used to calculate the last day of the fertile period, not the first.
· Shortest cycle -18 days: Correct answer; this method calculates the first day of the fertile window by accounting for cycle variability.
· Shortest cycle -10 days: Incorrect; this calculation is not used in determining the fertile period.