Background: Even though postpartum IUCD is effective, reliable, safe and recommended contraceptive method during postpartum women, only a small amount of mother who gave birth in health facilities used PPIUCD in the world. Besides the underlining problem and as to the knowledge of the researcher; nothing is known in Gamo zone about the determinants of PPIUCD. Therefore, this study aimed to assess determinants of postpartum IUCD utilization among mothers who gave birth in Gamo zone health facilities, southern Ethiopia, 2019. Methods: a facility based unmatched case control study was conducted in Gamo zone public health facilities, southern Ethiopia. A total of 536(179 cases and 357 controls) participants were selected in the study with a 1:2 case to control ratio. Data was collected by a pretested interviewer administered questionnaires from March 1 to April 15 2019. The data was coded, cleaned and entered by Epi-Info version 3.5.1 and exported to SPSS version 25 for advanced analysis. Bivariable and multivariable logistic regression analysis were performed to identify the determinants. Result: A total of 510(175 cases and 335 control) were involved in the study, which makes a response rate of 95.1%. The odds of mothers who have partner support for IUCD insertion were ten times more likely to utilize PPIUCD than those do not have partner support. (AOR (95 CI) 10(4.03, 24.3)). Mothers who have a birth interval of 1-2 year and 3-5 years 12 and almost 10 times more likely to utilize PPIUCD than mothers who have greater than five years. (AOR (95 CI) 12.0(1.5, 97.7), 9.7(1.7, 55.1)) respectively. The odds of mothers who are spacer four times more likely to utilize PPIUCD than limiters (AOR (95 CI) 4(1.44,10.84)). Mothers who are counseled during early labor around 88% less likely to utilize PPIUCD than those counseled during ANC (AOR (95 CI) .125(.034, .46)). Conclusion and recommendation : Birth interval, fertility plan, timing of counseling, pregnancy plan and partner support were determinant factors. Therefore, health professionals better to give counseling by incorporating partners during antenatal follow up and after delivery. Key words : Postpartum IUCD, Gamo zone, unmatched case control, determinant factors

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