• Z Li, E A Sullivan, M Chapman, C Farquhar, and Y A Wang.
• Faculty of Health, University of Technology Sydney, Sydney 2007, Australia School of Women's and Children's Health, The University of New South Wales, Sydney 2031, Australia Sydney Medical School, The University of Sydney, Sydney 2006, Australia.
• Hum. Reprod. 2015 Sep 1; 30 (9): 2048-54.

Study QuestionWhat type of transferred embryo is associated with a lower rate of ectopic pregnancy?Summary AnswerThe lowest risk of ectopic pregnancy was associated with the transfer of blastocyst, frozen and single embryo compared with cleavage stage, fresh and multiple embryos.What Is Known AlreadyEctopic pregnancy is a recognized complication following assisted reproductive technology (ART) treatment. It has been estimated that the rate of ectopic pregnancy is doubled in pregnancies following ART treatment compared with spontaneous pregnancies. However, it was not clear whether the excess rate of ectopic pregnancy following ART treatment is related to the underlying demographic factors of women undergoing ART treatment, the number of embryos transferred or the developmental stage of the embryo.Study Design, Size, DurationA population-based cohort study of pregnancies following autologous treatment cycles between January 2009 and December 2011 were obtained from the Australian and New Zealand Assisted Reproduction Technology Database (ANZARD). The ANZARD collects ART treatment information and clinical outcomes annually from all fertility centres in Australia and New Zealand.Participants/Materials, Setting, MethodsBetween 2009 and 2011, a total of 44 102 pregnancies were included in the analysis. The rate of ectopic pregnancy was compared by demographic and ART treatment factors. Generalized linear regression of Poisson distribution was used to estimate the likelihood of ectopic pregnancy. Odds ratios, adjusted odds ratios (AOR) and 95% confidence intervals (CI) were calculated.Main Results And The Role Of ChanceThe overall rate of ectopic pregnancy was 1.4% for women following ART treatment in Australia and New Zealand. Pregnancies following single embryo transfers had 1.2% ectopic pregnancies, significantly lower than double embryo transfers (1.8%) (P < 0.01). The highest ectopic pregnancy rate was 1.9% for pregnancies from transfers of fresh cleavage embryo, followed by transfers of frozen cleavage embryo (1.7%), transfers of fresh blastocyst (1.3%), and transfers of frozen blastocyst (0.8%). Compared with fresh blastocyst transfer, the likelihood of ectopic pregnancy was 30% higher for fresh cleavage stage embryo transfers (AOR 1.30, 95% CI 1.07-1.59) and was consistent across subfertility groups. Transfer of frozen blastocyst was associated with a significantly decreased risk of ectopic pregnancy (AOR 0.70, 95% CI 0.54-0.91) compared with transfer of fresh blastocyst.Limitations, Reason For CautionA limitation of this population-based study is the lack of information available on clinical- specific protocols and processes for embryo transfer (i.e. embryo quality, cryopreservation protocol, transfer techniques, etc.) and the potential impact on outcomes.Wider Implications Of The FindingsThe lowest risk of ectopic pregnancy was associated with the transfer of a single frozen blastocyst. This finding adds to the increasing evidence of better perinatal outcomes following frozen embryo transfers. The approach of freezing all embryos in the initiated fresh cycle and transfer of a single frozen blastocyst in the subsequent thaw cycle may improve the overall pregnancy and birth outcomes following ART treatment, in part by reducing the ectopic pregnancy rate.Study Funding/Competing InterestsThere is no funding for this study. Authors declared no competing interest related to this study.© The Author 2015. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

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