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We concluded a case-control study to explore this association by recruiting 2,411 EP patients as case group and 2,416 women with intrauterine pregnancy and 2,419 non-pregnant women as control groups. Odds ratios (ORs) and their 95% confidential intervals (CIs) were calculated and adjusted for potential confounding factors. Previous use of LNG-EC was not correlated with the EP. Compared to women who did not use contraceptives, current use of LNG-EC reduced the risk for intrauterine pregnancy (Adjusted OR [AOR] = 0.20, 95%CI: 0.14\u20130.27), but did not increase the risk for EP (AOR<jats:sub>2<\/jats:sub> = 1.04, 95%CI: 0.76\u20131.42). Furthermore, compared to women who did not have further act of intercourse, women with unprotected further act of intercourse were at a higher risk of EP (AOR<jats:sub>1<\/jats:sub> = 2.35, 95%CI: 1.17\u20134.71) and women with repeated use of LNG-EC for further intercourse during the same cycle was also associated with a higher risk for EP (AOR<jats:sub>1<\/jats:sub> = 3.08, 95%CI: 1.09\u20138.71; AOR<jats:sub>2<\/jats:sub> = 2.49, 95%CI: 1.00\u20136.19). A better understanding of the risk of EP following LNG-EC failure can optimize LNG-EC use and thus reduce the risk of EP.<\/jats:p>","DOI":"10.1038\/srep08487","type":"journal-article","created":{"date-parts":[[2015,2,12]],"date-time":"2015-02-12T14:02:48Z","timestamp":1423749768000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":27,"title":["Association between levonorgestrel emergency contraception and the risk of ectopic pregnancy: a multicenter case-control study"],"prefix":"10.1038","volume":"5","author":[{"given":"Jian","family":"Zhang","sequence":"first","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Cheng","family":"Li","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Wei-Hong","family":"Zhao","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Xiaowei","family":"Xi","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Shu-Jun","family":"Cao","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Hua","family":"Ping","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Guo-Juan","family":"Qin","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Linan","family":"Cheng","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"He-Feng","family":"Huang","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"297","published-online":{"date-parts":[[2015,2,12]]},"reference":[{"key":"BFsrep08487_CR1","doi-asserted-by":"publisher","first-page":"583","DOI":"10.1016\/S0140-6736(05)67103-6","volume":"366","author":"CM Farquhar","year":"2005","unstructured":"Farquhar, C. 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