Clinical trial registration was not an indicator for low risk of bias.

OBJECTIVES: To determine the prevalence of registered trials and to evaluate the risk of bias between registered and unregistered clinical trials.

STUDY DESIGN AND SETTING: 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.