Why is the 72-hour time frame critical for the action of emergency contraceptive pills, such as levonorgestrel and ulipristal acetate?

The 72-Hour Window for Emergency Contraceptive Pills

The 72-hour timeframe is critical for emergency contraceptive pills because effectiveness significantly decreases after this period, with pregnancy risk increasing approximately five times when taken on the fifth day compared to the first day after unprotected intercourse. 1

Mechanism and Timing Considerations

• Emergency contraceptive pills primarily work by delaying or inhibiting ovulation, and their effectiveness diminishes as time passes after unprotected intercourse 2
• Levonorgestrel (Plan B) should be taken within 72 hours (3 days) of unprotected intercourse for optimal effectiveness, though it can still provide some protection up to 120 hours afterward 2, 3
• When taken within the recommended 72-hour window, levonorgestrel reduces pregnancy risk by at least 75% 2
• The sooner levonorgestrel is taken after unprotected intercourse, the better it works, making immediate administration crucial 3

Effectiveness Timeline

• Effectiveness of emergency contraception decreases progressively with time:
  • Treatment initiated within 24 hours provides the highest effectiveness 2
  • Effectiveness remains relatively stable through 72 hours 2
  • Pregnancy rates increase significantly at 4-5 days after unprotected intercourse 2, 1
• Levonorgestrel's effectiveness drops substantially after 72 hours, with odds of pregnancy increasing nearly 6 times by the fifth day compared to administration within the first 24 hours 1

Alternative Emergency Contraception Options

• Ulipristal acetate (UPA) maintains effectiveness throughout a full 120-hour (5-day) window, making it a better option when more than 72 hours have passed 4, 5, 6
• In clinical trials, UPA prevented significantly more pregnancies than levonorgestrel when used between 72 and 120 hours after unprotected intercourse (0 pregnancies with UPA vs. 3 with levonorgestrel) 7
• The copper intrauterine device (Cu-IUD) is the most effective emergency contraception option and can be inserted up to 5-7 days after unprotected intercourse 4, 8

Clinical Implications

• The 72-hour window is emphasized in product labeling and clinical guidelines because:
  • It represents the timeframe with proven high effectiveness for levonorgestrel 3
  • It provides clear guidance for patients about when to seek emergency contraception 2
• For patients presenting after 72 hours:
  • Ulipristal acetate should be considered instead of levonorgestrel as it maintains better efficacy between 72-120 hours 4, 6
  • A copper IUD remains the most effective option if appropriate for the patient 4

Practical Recommendations

• Advance provision of emergency contraception may increase the likelihood of timely use within the critical 72-hour window 2, 4
• If vomiting occurs within 2-3 hours of taking emergency contraceptive pills, another dose should be taken as soon as possible 2, 3
• Regular contraceptive methods can be started immediately after using emergency contraception 2