We undertake a nationwide US study to estimate how mega-fires (defined as wildfires >100,000 acres in size) affect short-term infant health outcomes in communities located within the flame zone. This is the first study to look exclusively at mega-fires, which have unique characteristics compared to smaller wildfire events and are becoming more frequent in the US. We find that pregnant mothers in affected counties experience 0.8 percentage point greater instances of low birth weight and 1.2 percentage point greater instances of prematurity. The low birth weight finding is equivalent to what would be expected if a mother smoked approximately 6.2 cigarettes per day during pregnancy. Importantly, impacts are potentially non-linearly increasing in mega-fire size. Improved benefit-cost analyses are needed to account for mega-fire indirect impacts when making wildfire control and suppression decisions.