BACKGROUND: The impact of heat waves on perinatal outcomes has become a focus of attention. However, the association between heat waves and preterm birth (PTB), low birth weight (LBW), small for gestational age (SGA) and large for gestational age (LGA) remains controversial. Furthermore, whether residential greenness can mitigate the adverse effects of heat waves on birth outcomes still unclear. METHODS: This study employed a retrospective cohort design based on data from the ELEFANT project, conducted in Tianjin, China, spanning from 2011 to 2019. We assessed heat waves exposure by creating 12 definitions of extreme heat with varying relative temperatures (90th, 95th, 97.5th and 99th percentiles) and durations (at least 2, 3, or 4 consecutive days). Residential greenness levels were evaluated using the MODIS Normalized Difference Vegetation Index (NDVI). Cox proportional hazards regression models were fitted to estimate the risks of PTB, LBW, SGA and LGA in relation to heatwaves exposure and the multiplicative and additive interactions of heat waves and greenness on adverse birth outcomes across different pregnancy periods, including pre-pregnancy. RESULTS: Our findings indicate that heatwaves significantly increase the risks of PTB by 2.4%-15.6%, LBW by 7.6%-18 %, SGA by 3.6%-88.4%, and LGA by 2%-3.4%. The lack of residential greenness exhibited a synergistic interaction with heatwaves on PTB, SGA and LGA (RERI >
0), especially during milder heat events during entire pregnancy. For LBW, interactions between greenness and heatwaves were limited. Furthermore, we identified that heatwave was significantly associated with high risk of adverse birth outcomes among mothers living in rural areas. CONCLUSIONS: Prioritizing greenspace can help mitigate the effects of heatwaves, offering a viable and cost-effective approach to protecting maternal and fetal health.