Objective: We aim to assess which variables are associated with recruitment failure of RCTs, leading to an extension of the study period.. Design: Nationwide cohort study. Setting: A cohort of RCTs supported by the trial centre of the Dutch Consortium of Obstetrics and Gynaecology. Population: We included 83 RCTs that recruited patients between March 1 ^st 2003 and December 1 ^st 2023. Main outcome measures: Primary outcome was recruitment target not achieved within six months after the pre-planned recruitment period. Secondary outcomes were recruitment target not achieved within an extension period of at least twelve months and premature termination of the trial. Results: In total, 46 of 83 RCTs (55%) did not achieve their targeted recruitment within the pre-planned study period with a maximal extension period of 6 months. The most relevant indicators for recruitment failure in multivariable risk prediction modelling were presence of a no-treatment arm, a compensation fee of less than 200 euros per included patient, funding of less than 350.000 euros, while a preceding pilot study lowered this risk. Conclusions: We identified that the presence of a no-treatment arm, low funding and a low compensation fee per included patient were the most relevant risk factors for recruitment failure within the pre-planned period, while a preceding pilot study lowered this risk. Awareness of these indicators is important when designing future studies.