A woman with regular cycles uses the calendar (rhythm) method. Her shortest cycle is 26 days and her longest cycle is 32 days. Her fertile (unsafe) period is:

• A Day 6 to day 20 counting from the first day of menstruation
• B Day 10 to day 17 counting from the first day of menstruation
• C Day 14 to day 21 counting from the first day of menstruation
• D Day 8 to day 22 counting from the first day of menstruation ✓

Explanation

By Ogino-Knaus rules, the first unsafe day equals the shortest cycle minus 18 (26 minus 18 = day 8) and the last unsafe day equals the longest cycle minus 10 (32 minus 10 = day 22). This accounts for sperm survival up to 5 days and ovum survival up to 1 day plus variation in ovulation timing. Option B reflects an average cycle, not this woman's calculated range.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.