This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Demography and family planning, Subject name: Social & Preventive Medicine
Safe period is calculated by 
Select and explain. 
A. Length of menstrual cycle, B. Duration Date of ovulation, C. Length of ovulation, D. Length of luteal phase
A: Length of menstrual cycle
Answer: A: Length of menstrual cycle 
Explanation: The safe period is calculated by the length of menstrual cycle The shoest cycle minus 18 days gives the first day of feile period and longest cycle minus 10 days gives the last day of the feile period The couple should abstain from coitus during feile period Reference : Park&;s textbook of preventive and social medicine, 23rd edition, Page no: 507