On 23 October 2015, operators at the Aliso Canyon Natural Gas Storage field in Northern Los Angeles reported an uncontrolled natural gas leak. The blowout persisted for 112 days, releasing ~109,000 metric tons of methane into the atmosphere. Elevated air toxics and fine particle pollutant levels were also measured in nearby communities. We used California's birth records and a quasiexperimental design to assess whether pregnant women living in affected communities during the disaster experienced more adverse birth outcomes than expected. Overall, the prevalence of low birthweight and term low birthweight were 45 to 100% higher than expected among women living in the affected communities whose late pregnancy overlapped with the blowout. The strongest effects were observed among women living directly south and southwest of the facility. Furthermore, we observed a dose-response effect, where the odds of low birthweight were highest among women living closest to the well and attenuated out.