BackgroundBirth spacing is crucial for ensuring the health of mothers and their children, as well as determining population growth. Short birth intervals represent a universal public health problem associated with adverse maternal, fetal, neonatal, and child outcomes. However, there is limited information in the study area regarding the determinants of suboptimal birth spacing. Thus, this study aimed to identify the determinants of suboptimal spacing among women of reproductive age in the Adama district, Ethiopia.MethodsA community-based unmatched case-control study was conducted among 568 randomly selected reproductive-age women using the multi-stage sampling technique. Data were collected using an interviewer-administered, structured questionnaire. The collected data were entered into Epi Info version 7.2 and analyzed using SPSS version 26. Binary logistic regression analysis was used to model the association between suboptimal birth spacing and independent variables. Adjusted odds ratios with their 95% confidence intervals were calculated to determine the strength of the association. A p-value < 0.05 was considered to declare statistical significance.ResultEducational status (no formal education) (AOR = 2.40; 95% CI: 1.23–1.75), inadequate knowledge of optimal birth space (AOR = 2.60; 95% CI; 1.80–3.90), non-use of modern contraceptives (AOR = 3.00; CI: 1.90–4.20), short breastfeeding duration (AOR = 2.30; 95% CI: 1.50–3.40), and having female index child (AOR = 1.60; 95% CI: 1.13–2.50) were independent determinants of suboptimal birth spacing practice.ConclusionEncouraging women’s education, contraceptive use, and breastfeeding is crucial for birth spacing. Community health initiatives should also focus on preventing sex-based birth intervals.