<b><i>Introduction:</i></b> The use of benzodiazepines and/or z-drugs in women of childbearing age has increased. <b><i>Objective:</i></b> The aim of the study was to evaluate whether gestational benzodiazepine and/or z-drug exposure is associated with adverse birth and neurodevelopmental outcomes. <b><i>Methods:</i></b> A population-based cohort including mother-child pairs from 2001 to 2018 in Hong Kong was analysed to compare gestationally exposed and nonexposed children on the risk of preterm birth, small for gestational age, autism spectrum disorder (ASD), and attention-deficit/hyperactivity disorder (ADHD) through logistic/Cox proportional hazards regression with a 95% confidence interval (CI). Sibling-matched analyses and negative control analyses were applied. <b><i>Results:</i></b> When comparing gestationally exposed with gestationally nonexposed children, the weighted odds ratio (wOR) was 1.10 (95% CI = 0.97–1.25) for preterm birth and 1.03 (95% CI = 0.76–1.39) for small for gestational age, while the weighted hazard ratio (wHR) was 1.40 (95% CI = 1.13–1.73) for ASD and 1.15 (95% CI = 0.94–1.40) for ADHD. Sibling-matched analyses showed no association between gestationally exposed children and their gestationally nonexposed siblings for all outcomes (preterm birth: wOR = 0.84, 95% CI = 0.66–1.06; small for gestational age: wOR = 1.02, 95% CI = 0.50–2.09; ASD: wHR = 1.10, 95% CI = 0.70–1.72; ADHD: wHR = 1.04, 95% CI = 0.57–1.90). Similarly, no significant differences were observed when comparing children whose mothers took benzodiazepines and/or z-drugs during pregnancy to children whose mothers took benzodiazepines and/or z-drugs before but not during pregnancy for all outcomes. <b><i>Conclusions:</i></b> The findings do not support a causal relationship between gestational benzodiazepines and/or z-drugs exposure and preterm birth, small for gestational age, ASD, or ADHD. Clinicians and pregnant women should carefully balance the known risks of benzodiazepines and/or z-drugs use against those of untreated anxiety and sleep problems.