RESUMEN
BACKGROUND: Pre-registration of a clinical trial before the first participant is recruited can help to prevent selective outcome reporting and salami-slicing that can distort the evidence base for an intervention and result in people being offered care or treatment that is not effective. Rates of clinical trial registration in nursing and midwifery are low. AIM: To use a hypothetical example from midwifery practice to illustrate how selective outcome reporting and salami-slicing can distort the evidence base. FINDINGS: A trial of immersion in water during labour and birth is used to consider issues in outcome selection and how researchers may be drawn to switch primary outcomes or report different outcomes across multiple papers. DISCUSSION: In nursing and midwifery science, selective outcome and salami reporting are seemingly common. Prospective trial registration is intended to prevent these practices, enhancing the quality and integrity of the work. CONCLUSION: Clinical trials are a robust form of primary research evidence and directly impact clinical practice. Researchers must ensure their trials are correctly registered and editors need to reconcile submitted papers and registration entries as part of the review process.