From the Guidelines
The morning-after pill is not effective when taken 4 days before ovulation, as its primary mechanism of action is to prevent or delay ovulation, and taking it before ovulation would not provide any benefit in preventing pregnancy if ovulation were to occur shortly after. When considering emergency contraception, it's essential to understand how different methods work and their effectiveness based on the timing of ovulation and unprotected sex. According to 1, ulipristal acetate (UPA) and levonorgestrel ECPs have similar effectiveness when taken within 3 days after unprotected sexual intercourse, but UPA has been shown to be more effective than the levonorgestrel formulation 3–5 days after unprotected sexual intercourse. However, the key factor in the effectiveness of these medications is their ability to prevent or delay ovulation. If a woman is 4 days before expected ovulation and takes the morning-after pill, it may still be effective in preventing or delaying ovulation, thus preventing pregnancy. But, if ovulation has already occurred or is imminent, the effectiveness of emergency contraceptive pills (ECPs) like UPA or levonorgestrel in preventing pregnancy is significantly reduced because they do not prevent implantation of a fertilized egg. For situations where timing is critical, and the window for preventing ovulation has passed, other methods like the Cu-IUD, as mentioned in 1, can be effective as emergency contraception when inserted within 5 days of the first act of unprotected sexual intercourse, or even beyond 5 days after sexual intercourse if insertion does not occur more than 5 days after ovulation. Given the information and the primary goal of minimizing morbidity, mortality, and maximizing quality of life, the most effective approach would be to use emergency contraception as soon as possible after unprotected sex, ideally within the recommended timeframe for each method, and to consider the Cu-IUD for its extended window of effectiveness and potential for ongoing contraception.
From the FDA Drug Label
The likely primary mechanism of action of ulipristal acetate for emergency contraception is therefore inhibition or delay of ovulation; however, alterations to the endometrium that may affect implantation may also contribute to efficacy. Pharmacodynamic data showed that administration of ella to 34 women in the late follicular phase postponed follicular rupture for at least 5 days in all (100%) of 8 subjects who took ella before the luteinizing hormone (LH) surge and 11 (79%) of 14 subjects who took ella immediately before ovulation (when LH has already started to rise)
The morning after pill, also known as ulipristal acetate, may not work if taken 4 days before ovulation. The drug's primary mechanism of action is to inhibit or delay ovulation, and its effectiveness depends on the timing of administration in the menstrual cycle. While the drug can postpone follicular rupture when taken immediately before ovulation, its effectiveness when taken 4 days before ovulation is not explicitly stated in the provided drug labels 2 2. Therefore, no conclusion can be drawn about the drug's effectiveness in this specific scenario.
From the Research
Effectiveness of the Morning After Pill
The morning after pill, also known as emergency contraception, is used to prevent pregnancy after unprotected sex. The effectiveness of the morning after pill when taken 4 days before ovulation is supported by several studies:
- A pilot study published in 2007 3 found that levonorgestrel emergency contraception was highly effective when taken before ovulation, with no pregnancies observed among 34 women who took the pill before or on the day of ovulation.
- The study suggests that the mechanism of action of oral emergency contraception is to delay ovulation, and it is ineffective post-ovulation.
- Another study published in 2016 4 states that the mechanism of action of oral emergency contraception is to delay ovulation, which supports the idea that taking the pill before ovulation can be effective.
Comparison of Emergency Contraception Methods
Different emergency contraception methods have varying levels of effectiveness:
- A systematic review and meta-analysis published in 2025 5 found that the copper intrauterine device is more effective than methods containing levonorgestrel for emergency contraception.
- A study published in 2013 6 found that ulipristal acetate prevents almost two-thirds of pregnancies, while levonorgestrel prevents around 50% of expected pregnancies.
- A review published in 2017 7 states that ulipristal acetate is more effective than levonorgestrel and can be used up to five days after intercourse.
Factors Affecting Effectiveness
Several factors can affect the effectiveness of emergency contraception:
- A study published in 2016 4 found that women who weigh more than 75 kg or have a body mass index greater than 25 kg/m(2) may have a higher risk of unintended pregnancy when using oral levonorgestrel emergency contraception.
- The same study suggests that ulipristal acetate or copper intrauterine devices may be preferable in this setting.