AbstractWhile mosquito‐borne diseases are currently most prevalent in mid‐latitude countries, rising global temperatures could expand their range. This paper investigates whether one such disease, dengue, harms newborns. The empirical design exploits epidemiological patterns of the disease spreading. Dengue infection rates in the mother's municipality of residence which prevailed during the gestation period are instrumented with exogenous factors that determine dengue incidence in municipalities that have tight social connections to the maternal municipality. Using a large longitudinal dataset of Brazilian birth records, I find that a higher dengue rate during the third trimester of gestation has a detrimental effect on birth weight. In utero exposure to dengue also increases the probability of cesarean delivery and can lead to more serious consequences such as increased fetal and maternal mortality rates.