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<div> Introduction In developing countries like Kenya, addressing the high population growth rate necessitates a focus on early Postpartum Family Planning (PPFP) use. Despite the critical need for PPFP, few researchers explore the application of health behaviour change theories to enhance FP use among postpartum women. This study assesses the application of adapted Theory of Planned Behaviour (TPB) in predicting intention for early PPFP in postpartum women in Western Kenya. Methods This randomized control trial included pregnant women aged 15 to 49 attending Antenatal Care (ANC) clinics, randomly assigned to the "Nurses’ arm," "Community arm," or "Control arm." The intervention provided family planning (FP) counseling. Trained nurses and Community Health Workers (CHW) delivered counseling in their respective arms, while the control arm received routine care. Adapted TPB was integrated into client exit interviews to identify constructs influencing early PPFP intentions. Structural equation modeling (SEM) was used to predict the intention for early PPFP in the adapted TPB. Results The SEM was optimized with the removal of client knowledge on early PPFP. The final model retained satisfaction with PPFP counseling, perceived normative beliefs, attitude towards PPFP, behaviour control of PPFP choice, and perceived risk of early postpartum pregnancy. Only satisfaction with counseling (P = 0.001), perceived normative beliefs (P<0.0001), attitude towards PPFP (P<0.0001), and behaviour control of PPFP choice (P = 0.018) significantly influenced early PPFP intention. Conclusion The study demonstrates a viable application of the adapted TPB model in predicting early PPFP intention in an interventional study. Trial registration The study was registered by the Pan African Clinical Trial Registry on 03 July 2021 with a Trial Registration Number <a href="https://pactr.samrc.ac.za/TrialDisplay.aspx?TrialID=15984" target="_blank">PACTR202107891858045</a>. The trial was prospectively registered. </div>