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However, only a few studies reported an association between endometrial preparation protocols on EP and results were conflicting. A registry-based retrospective cohort study of 153,354 clinical pregnancies following frozen single ETs between 2014 and 2017 were conducted, of which 792 cases of EP (0.52%) were reported. Blastocyst embryo transfers accounted for 87% of the total sample and were significantly associated with a decreased risk for EP compared with early cleavage ET (0.90% vs. 0.46%, adjusted OR\u2009=\u20090.50, 95% CI, 0.41 to 0.60). Compared with natural cycles, hormone replacement cycles (HRC) demonstrated a similar risk for EP (0.53% vs. 0.47%, adjusted OR\u2009=\u20091.12, 95% CI, 0.89 to 1.42). Subgroup analysis with or without tubal factor infertility and early cleavage\/blastocyst ETs demonstrated similar non-significant associations. Endometrial preparation protocols using clomiphene (CC) were associated with a significantly increased risk for EP (1.12%, adjusted OR\u2009=\u20092.34; 95% CI, 1.38 to 3.98). These findings suggest that HRC and natural cycles had a similar risk for EP. Endometrial preparation using CC was associated with an increased risk of EP in frozen embryo transfer cycles.<\/jats:p>","DOI":"10.1038\/s41598-021-97044-6","type":"journal-article","created":{"date-parts":[[2021,8,31]],"date-time":"2021-08-31T10:03:02Z","timestamp":1630404182000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":10,"title":["Effect of endometrial preparation protocols on the risk of ectopic pregnancy for frozen embryo transfer"],"prefix":"10.1038","volume":"11","author":[{"given":"Seung Chik","family":"Jwa","sequence":"first","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Masashi","family":"Takamura","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Akira","family":"Kuwahara","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Takeshi","family":"Kajihara","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Osamu","family":"Ishihara","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"297","published-online":{"date-parts":[[2021,8,31]]},"reference":[{"key":"97044_CR1","doi-asserted-by":"publisher","first-page":"666","DOI":"10.1016\/j.fertnstert.2016.05.019","volume":"106","author":"L Londra","year":"2016","unstructured":"Londra, L., Moreau, C., Strobino, D., Bhasin, A. & Zhao, Y. 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