Effectiveness of Emergency Contraceptive Pills and Pregnancy Risk - Obstetrics and Gynecology

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Emergency contraceptive pills (ECPs), such as the one you mentioned (Levonorgestrel, commonly known as Plan B or in your case, "莉潔婷錠"), are designed to prevent pregnancy after unprotected intercourse or contraceptive failure. They are most effective when taken as soon as possible after the incident, ideally within 72 hours, but can be taken up to 120 hours (5 days) after unprotected sex, with decreasing effectiveness over time.

In your situation, since you took the emergency contraceptive pill approximately seven hours after the first instance of potential contraceptive failure, you are within the optimal time frame for the pill to be effective. The effectiveness of ECPs is around 75-89% when taken within the first 72 hours, and while the efficacy decreases after that, it still provides a significant chance of preventing pregnancy if taken within the 120-hour window.

Regarding the risk of pregnancy, it is important to consider a few factors:
1. Timing of Ovulation: If your girlfriend's menstrual cycle is regular and she ovulates around the middle of her cycle (which is typically around 14 days after the first day of her last period), the risk of pregnancy would depend on whether she was ovulating at the time of the unprotected intercourse. Since her last period started on April 21, ovulation would likely occur around May 5, making the risk of pregnancy lower if she had not yet ovulated.

2. Multiple Instances of Intercourse: You mentioned that there were two additional instances of protected intercourse after the first event. If the condom did not break and was used correctly, the risk of pregnancy from those encounters would be minimal.

3. ECP Mechanism: ECPs primarily work by delaying ovulation. If ovulation has already occurred, they may not be effective. However, since you took the pill shortly after the first incident, it is likely that it will prevent ovulation from occurring.

4. Menstrual Cycle Regularity: If your girlfriend has a regular cycle, it can help in predicting ovulation and understanding her risk of pregnancy. If her cycles are irregular, it may be more challenging to assess the risk accurately.

In summary, given that your girlfriend took the emergency contraceptive pill within a reasonable time frame after the potential contraceptive failure, the likelihood of pregnancy is significantly reduced. However, if her next period is delayed by more than a week, it would be advisable for her to take a pregnancy test to confirm whether or not she is pregnant.

Additionally, it is important to have open communication with your partner about contraceptive methods and to consider discussing long-term contraceptive options to avoid similar situations in the future. Emergency contraception is a useful option, but it should not be relied upon as a primary method of birth control. Regular contraceptive methods, such as hormonal birth control, IUDs, or condoms, can provide more reliable protection against unintended pregnancies.