IntroductionBirth spacing is a critical pathway to improving reproductive health. WHO recommends a minimum of 33-month interval between two consecutive births to reduce maternal, perinatal, infant morbidity and mortality. Our study evaluated factors associated with short birth intervals (SBIs) of less than 33 months between two consecutive births, in Karachi, Pakistan.MethodsWe used data from a cross-sectional study among married women of reproductive age (MWRA) who had at least one live birth in the 6 years preceding the survey (N=2394). Information regarding their sociodemographic characteristics, reproductive history, fertility preferences, family planning history and a 6-year reproductive calendar were collected. To identify factors associated with SBIs, we fitted simple and multiple Cox proportional hazards models and computed HRs with their 95% CIs.ResultsThe median birth interval was 25 months (IQR: 14–39 months), with 22.9% (833) of births occurring within 33 months of the index birth. Women’s increasing age (25–30 years (aHR 0.63 (0.53 to 0.75), 30+ years (aHR 0.29, 95% CI 0.22 to 0.39) compared with 20-24 years; secondary education (aHR 0.75, 95% CI 0.63 to 0.88), intermediate education (aHR 0.62, 95% CI 0.48 to 0.80), higher education (aHR 0.69, 95% CI 0.51 to 0.92) compared with no education, and a male child of the index birth (aHR 0.81, 95% CI 0.70 to 0.94) reduced the likelihood of SBIs. Women’s younger age <20 years (aHR 1.24, 95% CI 1.05 to 1.24) compared with 20–24 years, and those who did not use contraception within 9 months of the index birth had a higher likelihood for SBIs for succeeding birth compared with those who used contraception (aHR 2.23, 95% CI 1.93 to 2.58).ConclusionStudy shows that birth intervals in the study population are lower than the national average. To optimise birth intervals, programmes should target child spacing strategies and counsel MWRA on the benefits of optimal birth spacing, family planning services and contraceptive utilisation.