title: Clinical trial registration was not an indicator for low risk of bias.
abstract: OBJECTIVE
  To determine the prevalence of registered trials and to evaluate the risk of bias between registered and unregistered clinical trials.
  
  
  METHODS
  The Cochrane Gynecology and Fertility Group's specialized register was searched on November 5, 2015, for randomized controlled trials published from 2010 to 2014. Studies were selected if they had randomized women or men for fertility treatments, were published in full text and written in English. Two reviewers then independently assessed trial registration status for each trial, by searching the publication, trial registries, and by contacting the original authors.
  
  
  RESULTS
  Of 693 eligible randomized controlled trials, only 44% were found to be registered. Unregistered clinical trials had smaller sample sizes than registered trials (P < 0.001). A random subsample of 125 registered and 125 unregistered trials was assessed for risk of bias using five of the Cochrane Risk of Bias "domains." Registered and unregistered trials differed in their risk of bias for random sequence generation (P = 0.001), allocation concealment (P = 0.003), and selective reporting (P < 0.001) but not blinding or incomplete outcome data (P > 0.05) domains. Only 54 (43.2%) of the 125 registered trials were registered prospectively. This study has the following limitations. Only English language trials were included in this review. We were unable to obtain protocols for the unregistered trials and therefore were unable to assess the risk of bias in the selective reporting domain.
  
  
  CONCLUSIONS
  All available trials should be included in systematic reviews and assessed for risk of bias as there are both registered trials with high risk of bias and unregistered trials with low risk of bias and by excluding unregistered trials more than half of the available evidence will be lost.
authors.affiliation: Department of Obstetrics and Gynaecology, Private Bag 92019, University of Auckland, Auckland 1142, New Zealand. Electronic address: c.farquhar@auckland.ac.nz.
authors.forename: Cynthia M
authors.initials: CM
authors.lastname: Farquhar
authors.affiliation: Department of Obstetrics and Gynaecology, Private Bag 92019, University of Auckland, Auckland 1142, New Zealand.
authors.forename: Marian G
authors.initials: MG
authors.lastname: Showell
authors.affiliation: Department of Obstetrics and Gynaecology, Private Bag 92019, University of Auckland, Auckland 1142, New Zealand.
authors.forename: Emily A E
authors.initials: EAE
authors.lastname: Showell
authors.affiliation: Department of Obstetrics and Gynaecology, Private Bag 92019, University of Auckland, Auckland 1142, New Zealand.
authors.forename: Penny
authors.initials: P
authors.lastname: Beetham
authors.affiliation: University of Groningen, Hanzeplein 1, 9713 GZ Groningen, The Netherlands.
authors.forename: Nora
authors.initials: N
authors.lastname: Baak
authors.affiliation: Department of Obstetrics and Gynaecology, Radboud University Medical Centre, P.O. Box 9101, Nijmegen 6500 HB, The Netherlands.
authors.forename: Selma
authors.initials: S
authors.lastname: Mourad
authors.affiliation: Department of Obstetrics and Gynaecology, Private Bag 92019, University of Auckland, Auckland 1142, New Zealand.
authors.forename: Vanessa M B
authors.initials: VMB
authors.lastname: Jordan
mesh_terms: D015982:Bias; D015340:Epidemiologic Research Design; D005260:Female; D006801:Humans; D007246:Infertility / Q000453:epidemiology* / Q000628:therapy; D008297:Male; D016032:Randomized Controlled Trials as Topic / Q000706:statistics & numerical data*; D012042:Registries / Q000706:statistics & numerical data*; D012306:Risk
keywords: Clinical trial registration; Evidence; Meta-analysis; Quality; Risk of bias; Systematic review