BACKGROUND: Left unabated, rising temperatures pose an escalating threat to human health. The potential effects of hot temperatures on fetal health have been under-explored. Here, we examined the association between prenatal ambient temperature exposure and fetal growth measures in a Massachusetts-based pregnancy cohort. METHODS: We used ultrasound measurements of biparietal diameter (BPD), head circumference (HC), femur length and abdominal circumference (AC), in addition to birthweight (BW), from 9446 births at Beth Israel Deaconess Medical Center from 2011 to 2016. Ultrasound scans were classified into three distinct gestational periods: 16-23 weeks, 24-31 weeks, 32+ weeks; and z-scores were created for each fetal growth measure using the INTERGROWTH-21st standards. We fitted distributed lag models to estimate the time-varying association between weekly temperature and fetal growth, adjusting for sociodemographic characteristics, seasonal and long-term trends, humidity and particulate matter (PM2.5). RESULTS: Higher ambient temperature was associated with smaller fetal growth measures. The critical window of exposure appeared to be Weeks 1-20 for ultrasound parameters, and high temperatures throughout pregnancy were important for BW. Associations were strongest for head parameters (BPD and HC) in early to mid-pregnancy, AC late in pregnancy and BW. For example, a 5ºC higher cumulative temperature exposure was associated with a lower mean AC z-score of -0.26 (95% CI: -0.48, -0.04) among 24-31-Week scans, and a lower mean BW z-score of -0.32 (95% CI: -0.51, -0.12). CONCLUSION: Higher temperatures were associated with impaired fetal growth. This has major health implications given that extreme temperatures are more common and escalating.