Preterm birth is a leading cause of infant deaths and illness in the U.S.—yet its underlying molecular causes remain largely unclear. About 40 to 50% of preterm births, defined as births before 37 weeks of pregnancy, are estimated to be “idiopathic,” meaning they arise from unexplained or spontaneous labor. And, maternal stress linked to depression and post-traumatic stress disorders as well as fetal stress have been strongly implicated in preterm births with no known cause.