Long-term climate change has increased the frequency and intensity of high-temperature weather events, which in turn affect human health and wellbeing. Warmer temperatures are reported to affect both reproductive health and behaviors, possibly reducing birth rates. In a low-fertility context, the potential negative impact that climate change might have on fertility is consequential. This study focuses on Italy, a low-fertility country disproportionately affected by climate change, with sharp regional disparities in both climate zones and economic development. Matching monthly birth registration data for the period 2003 to 2022 with E-OBS, ERA-5 and CAMS meteorological data, we analyze the relationship between heat exposure and total fertility rates in 107 Italian provinces (corresponding to the NUTS-3 classification). Results show that exposure to hot days, which are defined as days with a mean temperature above 25 °C, has a relatively immediate impact on conception probabilities as it reduces the total fertility rate nine months later. Additional analyses revealed that the largest effect of heat on fertility occurs in above-average GDP provinces and provinces with lower-than-average healthcare workforce availability.