OBJECTIVE: To quantify the short-term effects of maternal exposure to heatwave on preterm birth. DESIGN: An ecological study. SETTING: A population-based study in Brisbane, Australia. POPULATION: All pregnant women who had a spontaneous singleton live birth in Brisbane between November and March in 2000-2010 were studied. METHODS: Daily data on pregnancy outcomes, meteorological factors, and ambient air pollutants were obtained. The Cox proportional hazards regression model with time-dependent variables was used to examine the short-term impact of heatwave on preterm birth. A series of cut-off temperatures and durations were used to define heatwave. Multivariable analyses were also performed to adjust for socio-economic factors, demographic factors, meteorological factors, and ambient air pollutants. MAIN OUTCOME MEASURE: Spontaneous preterm births. RESULTS: The adjusted hazard ratios (HRs) ranged from 1.13 (95% CI 1.03-1.24) to 2.00 (95% CI 1.37-2.91) by using different heatwave definitions, after controlling for demographic, socio-economic, and meteorological factors, and air pollutants. CONCLUSIONS: Heatwave was significantly associated with preterm birth: the associations were robust to the definitions of heatwave. The threshold temperatures, instead of duration, could be more likely to influence the evaluation of birth-related heatwaves. The findings of this study may have significant public health implications as climate change progresses.