Is levonorgestrel (emergency contraceptive) effective in preventing pregnancy if ovulation has already occurred during intercourse?

Levonorgestrel Effectiveness After Ovulation

Levonorgestrel is NOT effective if ovulation has already occurred at the time of intercourse, and you should instead offer ulipristal acetate (UPA) or a copper IUD for emergency contraception in this scenario. 1

Mechanism of Action Explains the Limitation

• Levonorgestrel works primarily by delaying or inhibiting ovulation—it does NOT prevent fertilization or implantation once ovulation has occurred 2, 3
• The FDA label explicitly states that "levonorgestrel tablet works before release of an egg from the ovary" and "usually stops or delays the release of an egg from the ovary" 2
• A pilot study directly tested this question and found that among 17 women who took levonorgestrel AFTER ovulation had already occurred (Days +1 to +2), the observed pregnancy rate matched the expected rate without any contraception (3 pregnancies observed vs. 3-4 expected), demonstrating zero effectiveness post-ovulation 1
• In contrast, among 34 women who took levonorgestrel BEFORE ovulation (Days -5 to -2), zero pregnancies occurred when 4 were expected, confirming high effectiveness when given pre-ovulation 1

Clinical Algorithm for Emergency Contraception Selection

When ovulation timing is uncertain (most clinical scenarios):

• Within 72 hours of intercourse: Levonorgestrel 1.5 mg is acceptable for normal-weight women, but UPA 30 mg is superior, particularly if the woman may be near ovulation 4, 5, 6
• Between 72-120 hours: UPA 30 mg is the only effective oral option, as levonorgestrel effectiveness drops significantly after 72 hours 4, 5, 7
• Any timepoint within 5 days: Copper IUD is the most effective method (<1% failure rate) regardless of ovulation timing and provides ongoing contraception 4, 5, 7

When ovulation has likely occurred or is imminent:

• Do NOT use levonorgestrel—it will be ineffective 1
• First choice: Copper IUD insertion within 5 days of intercourse (or up to 5 days after ovulation if timing can be estimated) 4, 7
• Second choice: UPA 30 mg, though its post-ovulatory effectiveness is uncertain, it is still superior to levonorgestrel 6

Weight Considerations

• Levonorgestrel may be less effective in women weighing >165 pounds regardless of ovulation timing 5
• For women >165 pounds, UPA is more effective than levonorgestrel even within the 72-hour window 5
• The copper IUD remains highly effective regardless of patient weight 5, 7

Critical Timing After Emergency Contraception Use

• After levonorgestrel: Resume regular contraception immediately but use barrier method or abstain for 7 days 4, 7
• After UPA: Start regular contraception immediately but use barrier method or abstain for 14 days (or until next menses) due to UPA's antiprogestin properties that could theoretically reduce hormonal contraception effectiveness 4, 7
• Advise pregnancy test if withdrawal bleeding does not occur within 3 weeks 4

Common Pitfall to Avoid

• Do not assume levonorgestrel works at all stages of the menstrual cycle—this is the most critical misconception. The drug's mechanism is purely pre-ovulatory, and clinical evidence confirms zero effectiveness post-ovulation 2, 3, 1
• Women often cannot accurately self-report their cycle stage, with major discrepancies found between self-report and endocrine dating 1
• When in doubt about ovulation timing, default to copper IUD or UPA rather than levonorgestrel to maximize pregnancy prevention 5, 7