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Background The effect of heatwaves on adverse birth outcomes is not well understood and may vary by how heatwaves are defined. Objectives To examine acute associations between various heatwave definitions and preterm and early-term birth in 53 metropolitan areas in the U.S during the 1980s. Methods Using national vital records from 53 metropolitan areas between 1982 to 1988, singleton preterm (<37 weeks) and early-term births (37-38 weeks) were matched (1:1) to controls who completed at least 37 weeks or 39 weeks of gestation, respectively. Matching variables included location, maternal race, and maternal education. 40 heatwave definitions were created based on temperature and humidity data, including binary indicators for exposure to sustained heat, number of high heat days, and measures of heat intensity (average degrees over threshold) based on the 97.5%ile of area-specific temperature and apparent temperature, or 85%ile of positive excessive heat factor (EHF). Heatwave exposure in the 7-days preceding birth (for cases) and corresponding gestational window (for controls) was assessed in relation to preterm and early-term birth. Odds ratios for heatwave exposures were estimated using conditional logistic regression adjusting for maternal age, marital status, and seasonality. Results There were a total of 615,329 and 1,293,813 preterm and early-term births in the analyses. Across heatwave definitions, exposure to heatwaves in the week before delivery was consistently associated with increased odds of early-term birth. Odds ratios increased across categories of increasing number of hot days, and minimum consecutive days of heat. For example, exposure to ≥3 days over 97.5%ile of apparent mean temperature in the past week compared to zero days, was associated with a 3.8% (95%CI: 2.6%-5.1%) increase in the odds of early-term birth. Effect estimates for preterm birth were generally consistent with a null association. Conclusions Preliminary results suggest heatwaves were associated with early-term birth but not preterm birth.