Randomized controlled trials (RCT) are widely considered to be the gold standard for demonstrating intervention effects. Adequacy of reporting of participant compliance in RCTs affects the interpretation of study results. Our aims were two-fold: first, to assess the adequacy of reporting of participant compliance in RCTs investigating the effect of maternal nutritional supplements on infant outcomes; and second, to examine authors' adherence to the Consolidated Standards of Reporting Trials (CONSORT) guidelines on participant flow. Papers reporting trials of nutritional supplementation during pregnancy, and published after revision of the CONSORT recommendations, were identified using a search of medical databases. Two researchers systematically reviewed the papers to assess the reporting of participant compliance according to specified criteria, and the presentation of participant flow data recommended in the CONSORT guidelines. The literature search identified 58 papers. Almost a third (n = 18) did not describe how participant compliance was assessed. Nearly half of the papers (n = 27) failed to report participant compliance numerically (absolute numbers or percentage) and differences in compliance data between treatment arms were not reported in 52% of papers (n = 28). The majority (83%) gave no information on whether the study protocol included any researcher input aimed at maximizing compliance. In addition to inadequate reporting of compliance, two of the CONSORT requirements (eligibility criteria and numbers discontinuing the intervention) were inadequately reported in 69% and 60% of papers, respectively. We conclude that participant compliance in nutrition trials is frequently inadequately reported. 'False negative' results from RCTs with poor compliance could wrongly influence policy and inhibit further research concerned with nutritional supplementation for women of child-bearing age. We suggest that changes to the CONSORT guidelines may improve RCT reporting.