53Identification of pregnancies at risk of preterm birth (PTB), the leading cause of newborn deaths, 54 remains challenging given the syndromic nature of the disease. We report a longitudinal multi-55 omics study coupled with a DREAM challenge to develop predictive models of PTB. We found 56 that whole blood gene expression predicts ultrasound-based gestational ages in normal and 57 complicated pregnancies (r=0.83), as well as the delivery date in normal pregnancies (r=0.86), 58with an accuracy comparable to ultrasound. However, unlike the latter, transcriptomic data 59 collected at <37 weeks of gestation predicted the delivery date of one third of spontaneous (sPTB) 60 cases within 2 weeks of the actual date. Based on samples collected before 33 weeks in 61 asymptomatic women we found expression changes preceding preterm prelabor rupture of the 62 membranes that were consistent across time points and cohorts, involving, among others, 63 leukocyte-mediated immunity. Plasma proteomic random forests predicted sPTB with higher 64 accuracy and earlier in pregnancy than whole blood transcriptomic models (e.g. AUROC=0.76 vs. 65 AUROC=0.6 at 27-33 weeks of gestation). 66 67 68 69 Early identification of patients at risk for obstetrical disease is required to improve health outcomes 70 and develop new therapeutic interventions. One of the "great obstetrical syndromes" 1 , preterm 71 birth, defined as birth prior to the completion of 37 weeks of gestation, is the leading cause of 72 newborn deaths worldwide. In 2010, 14.9 million babies were born preterm, accounting for 11.1% 73 of all births across 184 countries-the highest preterm birth rates occurring in Africa and North 74 America 2 . In the United States, the rate of prematurity remained fundamentally unchanged in 75 recent years 3 and it has an annual societal economic burden of at least $26.2 billion 4 . The high 76 incidence of preterm birth is concerning: 29% of all neonatal deaths worldwide, approximately 1 77 million deaths in total, can be attributed to complications of prematurity 5 . Furthermore, children 78 born prematurely are at increased risk for several short-and long-term complications that may 79 include motor, cognitive, and behavioral impairments 6,7 . 80Approximately one-third of preterm births are medically indicated for maternal (e.g. preeclampsia) 81 or fetal conditions (e.g. growth restriction); the other two-thirds are categorized as spontaneous 82 preterm births, inclusive of spontaneous preterm labor and delivery with intact membranes (sPTD), 83and preterm prelabor rupture of the membranes (PPROM) 8 . Preterm birth is a syndrome with 84 multiple etiologies 9 , and its complexity makes accurate prediction by a single set of biomarkers 85 difficult. While genetic risk factors for preterm birth have been reported 10 , the two most powerful 86 predictors of spontaneous preterm birth are a sonographic short cervix in the midtrimester, and a 87 history of spontaneous preterm birth in a prior pregnancy. 11 As for prevention of the syndrome, 88vagi...