Understanding the Pearl Index

The Pearl Index expresses contraceptive failure as the number of unintended pregnancies per 100 woman-years of use. A higher Pearl Index means a less effective method, since more failures occur per 100 women using it for a year.

Comparing the Options

• Calendar rhythm method: This natural method relies on predicting the fertile window from menstrual cycle calculations alone, with no barrier or hormonal backup. Because ovulation timing varies cycle to cycle and user error is common, this method has the highest Pearl Index (roughly 9–25) among conventional contraceptive methods, making it the least reliable of the options listed.
• Intrauterine contraceptive devices (IUDs): Copper and hormonal IUDs are highly effective, long-acting, and not dependent on user compliance, giving a very low Pearl Index (typically well under 1).
• Barrier contraceptives: Methods such as condoms and diaphragms have a moderate Pearl Index (around 2–18 depending on consistency of use), which is generally lower than that of the rhythm method.
• Combined oral contraceptives (COCs): With perfect use, the Pearl Index is under 1; even with typical use (accounting for missed pills), it remains lower than that of the rhythm method (around 0.3–9).

Conclusion

Among all the options, the calendar rhythm method has the highest failure rate and therefore the highest Pearl Index, as it depends entirely on accurate cycle prediction with no mechanical or hormonal safeguard against error.

Hence, the correct answer is calendar rhythm method.