**Background:** Short Birth Interval (SBI), defined as an interval of less than 33 months between two subsequent births, is highly prevalent in Low- and Middle-Income Countries (LMICs) including Bangladesh. Previous studies comparing socio-economically unequal groups may have overestimated or underestimated the true effect of SBI on child mortality. This study aimed to determine the effects of SBI on neonatal, infant, and under-five mortality in Bangladesh by comparing two equal groups created through propensity score matching.
**Methods:** The study analyzed linked data from the 2017/18 Bangladesh Demographic and Health Survey (BDHS) and the 2017 Bangladesh Health Facility Survey (BHFS). The BDHS used a two-stage stratified cluster sampling design, selecting 672 clusters and 30 households per cluster, yielding 20,127 interviewed women (response rate 98.8%). The BHFS included 1,524 health facilities. The final analytic sample comprised 5,941 women who had at least two pregnancies, with the most recent ending in live birth within five years. The exposure variable was SBI (<33 months between subsequent births). Outcome variables were neonatal mortality (within 28 days), infant mortality (within 1 year), and under-five mortality (within 5 years). Covariates were selected using a Directed Acyclic Graph (DAG) and included maternal age at birth, maternal education, maternal formal employment, partner's education, sex of household head, total children ever born, exposure to mass media, and sex of the deceased child's sibling. Propensity score matching with inverse probability of treatment weighting was applied, and multilevel Poisson regression was used to estimate adjusted prevalence ratios (aPR) with 95% confidence intervals.
**Key Results:** The mean age of mothers was 25.93 years (SD ±5.13), and mean years of education was 6.12 (SD ±3.70). Approximately 26.36% of mothers had short interval births. The prevalence rates of neonatal, infant, and under-five mortality were 23.1, 30.8, and 48.8 per 1000 live births, respectively. After propensity score weighting, standardized differences of all covariates were <1%, indicating good balance between groups. Newborns of SBI mothers had a 63% higher likelihood of neonatal mortality (aPR, 1.63; 95% CI, 1.08–2.46) compared to newborns of non-SBI mothers. Infant mortality was 1.45 times higher (aPR, 1.45; 95% CI, 1.01–2.08) among babies born in short intervals. The strongest effect was for under-five mortality, which was 2.82 times higher (aPR, 2.82; 95% CI, 2.16–3.70) among children born in short intervals compared to those born in normal intervals.
**Clinical Implications:** SBI is a strong predictor of child mortality in Bangladesh, with approximately 1 in 4 children born in short intervals at risk of dying before age five. The mechanisms include maternal folate depletion, nutritional deficiencies, uterine rupture, postpartum hemorrhage, preterm birth, low birth weight, and increased sibling competition for care and nutrition. With current neonatal and under-five mortality rates at 17 and 29 per 1000 live births respectively, and around 4 million yearly births, Bangladesh faces challenges in achieving SDG targets (12 neonatal and 25 under-five deaths per 1000 live births by 2030). The study recommends awareness-building programs about adverse effects of SBI, strengthening healthcare facilities, ensuring family planning services and contraception accessibility, and improving community-level education, particularly for socio-economically disadvantaged women who are most affected by short birth intervals.