Differences of opinion were resolved after team discussion. Two review authors (AM, SP) independently conducted the searches and selected the studies to be included. Cross references from the included studies were hand searched. Relevant journals in the specialty (Human Reproduction, Human Reproduction Update, RBM online and Fertility and Sterility) were searched electronically. Data sources and searchesĪ literature search with no language restrictions was performed (1984–2016) on Medline, EMBASE, Cochrane Central Register of Clinical Trials, CINAHL and DARE ( Supplementary Table S1).

The protocol was registered at PROSPERO (CRD42016046131). PRISMA guidelines for systematic reviews were followed ( ). We therefore conducted a new systematic review incorporating all the published studies and including a cumulative meta-analysis to examine whether the results have remained consistent over time. Subsequent primary studies ( Chen et al., 2016 Maheshwari et al., 2016) are beginning to challenge some of these initial findings. Initial observational studies and a systematic review based on these which was published 5 years ago, have suggested that obstetric and perinatal outcomes are better in those conceived following frozen rather than fresh embryo transfers ( Maheshwari et al., 2012), with reduced risks of preterm birth, small for gestational age babies, low birth weight babies and pre-eclampsia. With refinement of technology over the last few decades, the number of frozen embryo transfers has increased as have pregnancy rates which, according to some authors, are better than those following the transfer of fresh IVF embryos ( Chen et al., 2016). The first live birth following the transfer of thawed cryopreserved embryos was reported in 1984. Cryopreserved embryos are usually thawed and replaced in a natural or hormonally manipulated cycle in women in whom a fresh embryo transfer fails to result in a pregnancy or in those who return for a second baby. Conventionally, embryos created by this process are transferred within the uterus after 2–5 days in culture, while any remaining embryos are frozen for subsequent use. In-vitro fertilisation involves hormonal stimulation of ovaries followed by surgical retrieval of oocytes and their insemination in the laboratory. IVF, ICSI, obstetric outcomes, perinatal outcomes, frozen replacement cycles, preterm delivery, fresh embryo transfer, cryopreservation, large for gestational age, small for gestational age Introduction

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