title: What is the contribution of embryo-endometrial asynchrony to implantation failure?
abstract: OBJECTIVE
  The synchronized development of a viable embryo and a receptive endometrium is critical for successful implantation to take place. The aim of this paper is to review current thinking about the importance of embryo-endometrial synchrony in in vitro fertilization (IVF).
  
  
  METHODS
  Detailed review of the literature on embryo-endometrial synchrony.
  
  
  RESULTS
  By convention, the time when the blastocyst first attaches and starts to invade into the endometrium has been defined as the 'window of implantation'. The term window of implantation can be misleading when it is used to imply that there is a single critical window in time that determines whether implantation will be successful or not. Embryo maturation and endometrial development are two independent continuous processes. Implantation occurs when the two tissues fuse and pregnancy is established. A key concept in understanding this event is developmental 'synchrony', defined as when the early embryo and the uterus are both developing at the same rate such that they will be ready to commence and successfully continue implantation at the same time. Many different events, including controlled ovarian hyperstimulation as routinely used in IVF, can potentially disrupt embryo-endometrial synchrony. There is some evidence in humans that implantation rates are significantly reduced when embryo-endometrial development asynchrony is greater than 3 days (±1.5 days).
  
  
  CONCLUSIONS
  Embryo-endometrial synchrony is critical for successful implantation. There is an unmet need for improved precision in the evaluation of endometrial development to permit better synchronization of the embryo and the endometrium prior to implantation.
authors.affiliation: Department of Obstetrics and Gynaecology, University of Melbourne, The Royal Women's Hospital, 20 Flemington Road, Parkville, 3052, VIC, Australia. wantinn.teh@mivf.com.au.|Reproductive Services, The Royal Women's Hospital, Parkville, VIC, Australia. wantinn.teh@mivf.com.au.
authors.forename: Wan-Tinn
authors.initials: WT
authors.lastname: Teh
authors.affiliation: Reproductive Services, The Royal Women's Hospital, Parkville, VIC, Australia.
authors.forename: John
authors.initials: J
authors.lastname: McBain
authors.affiliation: Department of Obstetrics and Gynaecology, University of Melbourne, The Royal Women's Hospital, 20 Flemington Road, Parkville, 3052, VIC, Australia.
authors.forename: Peter
authors.initials: P
authors.lastname: Rogers
mesh_terms: D001755:Blastocyst / Q000166:cytology; D010064:Embryo Implantation / Q000235:genetics* / Q000502:physiology; D004624:Embryo Transfer; D047108:Embryonic Development / Q000235:genetics*; D004717:Endometrium / Q000254:growth & development*; D005260:Female; D005307:Fertilization in Vitro; D006801:Humans; D011247:Pregnancy; D014599:Uterus / Q000254:growth & development
keywords: Embryo; Endometrium; Implantation; Synchrony; Uterine receptivity