Background: Birth spacing is a key in ensuring the health of mothers and their children as well as determining population growth. Most of mothers in developing nations including Ethiopia have been practicing short birth interval. There is a paucity of studies concerned with birth interval among women in reproductive age in Ethiopia, specifically in the study area.

Objective: this study was aimed to identify the determinants of birth interval practice among reproductive age women in Gedeo zone, south Ethiopia.

Methods: A community based unmatched case-control study was undertaken among 814 reproductive age women in Gedeo zone, south Ethiopia from August 1 to September 30, 2018. Cases were women practiced short birth intervals (<33 months), whereas controls were women practiced birth intervals of 33 months and more. Structured interviewer-administered questionnaire was used. Stratified, two-stage cluster sampling technique was used. EpiData version 3.1 and SPSS version 23 were used for data entry and analysis, respectively. Bivariate and multivariable logistic regression analysis were computed. P value < 0.05 was considered as statistically significant. All ethical procedures were considered.

Results: Ag at first marriage, AOR (95%CI) 1.1(1.01, 1.17), distance from the nearest health facility, AOR (95%CI) 0.98(0.97, 0.99), wealth index, AOR (95%CI) 3.7(2.52, 5.55), and postnatal care utilization after the previous birth, AOR (95%CI) 3.0(2.06, 4.37) were statistically significant with birth interval practice.

Conclusion: Ag at first marriage, distance from the nearest health facility, wealth index and postnatal care utilization after the previous birth were the determinants of birth interval practice.