Background <p>The Emergency contraception pills (ECPs) are being used worldwide to prevent unwanted pregnancy, but the most important issue is still unanswered: whether they are effective when taken during or after ovulation, especially when taken near the luteinizing hormone (LH) surge. The knowledge of the time of efficacy and mechanism of action is essential to proper patient counseling, shared decision-making, and informed choice of contraceptives.</p> Methods <p>It is a This systematic review (no meta-analysis) is written in accordance with PRISMA 2020 and is registered in the prospective in PROSPERO (CRD420251238977). A careful search of PubMed/MEDLINE, Embase, Scopus, Cochrane Central, ClinicalTrials.gov, ProQuest Dissertations, and WHO ICTRP (January 2000-November 2025) was carried out to find articles that reported the outcome of pregnancy, ovulation outcome, or the mechanism of such drugs levonorgestrel and/or ulipristal acetate and/or mifepristone. The study designs that were included were 13 randomized controlled trials, 8 prospective cohort studies, 6 pharmacodynamic trials and 6 mechanistic studies. Cochrane RoB 2.0, the Newcastle-Ottawa Scale, or an established pharmacologic appraisal tool was used to assess risk of bias, and certainty of evidence was assessed using GRADE.</p> Findings <p>33 articles (<i>n</i> = 5,382 women) were included. In line with our interest, the results are arranged in terms of time with respect to ovulation. The effect of levonorgestrel (LNG) declined significantly with the increase in the dose to ovulation. Pregnancy rates when LNG was given &gt; 72&#xa0;h before expected ovulation were also low at only 0.6–1.8%. Effectiveness dropped as timing got to ovulation: at 24–72&#xa0;h before ovulation, it increased to 2.0-8.3%; during the peri-ovulatory period, it rose to 18.5–35.2%. The LH surge became even more fertile with a pregnancy rate of 40.3–59.8%, which was similar to uninformed cycles. Ulipristal acetate (UPA) was also found to be significantly less effective than LNG in the peri-ovulatory window (8.2–15.3%) and was also capable of delaying ovulation in approximately 60–78 per cent of users, but was not capable of doing so once ovulation had occurred (29–37% pregnancy). A mainly anti-ovulatory effect was established by mechanical research: at therapeutic doses, neither LNG nor UPA produced statistically significant effects on fertilization, implantation, or early embryo viability.</p> Conclusions <p>This systematic review indicates that the timing of ovulation plays a key role in determining the effectiveness of oral emergency contraceptive pills. It has been found that levonorgestrel is very effective if taken before the luteinizing hormone surge, whereas ulipristal acetate can possibly maintain its effectiveness till the early peri, ovulatory phase. The evidence from the studies, however, does not show a significant pregnancy risk reduction when these drugs are taken after ovulation, but it is not very certain due to ethical and methodological limitations in this phase. Since this review is based on the qualitative synthesis without the meta, analysis, the findings should be considered as descriptive trends rather than the quantitative effect estimations. The outcomes of this review advocate cycle, stage informed counseling and align with the present clinical guidelines that recommend the copper intrauterine device as the most effective emergency contraceptive method when ovulation is likely to have already taken place.</p>