Abstract Background The preconception period provides a window of opportunity for interventions aiming to reduce unhealthy lifestyle behaviours and their negative effect on pregnancy outcomes. This study aimed to assess the effectiveness of a locally tailored preconception care (PCC) intervention in a hybrid-II effectiveness implementation design. Methods A stepped-wedge cluster randomized controlled trial was performed in four Dutch municipalities. The intervention contained a social marketing strategy aiming to improve the uptake (prospective parents) and the provision (healthcare providers) of PCC. Prospective parents participated by administering a questionnaire in early pregnancy recalling their preconceptional behaviours. Experiences of healthcare providers were also evaluated through questionnaires. The composite primary outcome was adherence to at least three out of four preconceptional lifestyle recommendations (early initiation of folic acid supplements, healthy nutrition, no smoking or alcohol ...