1Background: Placental protein expression plays a crucial biological role during normal and 2 complicated pregnancies. We hypothesized that: (1) circulating pregnancy-associated, placenta-3 related protein levels throughout gestation reflect the uncomplicated, full-term temporal 4 progression of human gestation, and effectively estimates gestational ages (GAs); (2) 5 pregnancies with underlying placental pathology, such as preeclampsia (PE), are associated with 6 disruptions in this GA estimation in early gestation; (3) malfunctions of this GA estimation can 7 be employed to identify impending PE. In addition, to explore the underlying biology and PE 8 etiology, we set to compare protein gestational patterns of human and mouse, using pregnant 9 heme oxygenase-1 (HO-1) heterozygote (Het) mice, a mouse model reflecting PE-like 10 symptoms. 11Methods: Serum levels of circulating placenta-related proteins -leptin (LEP), chorionic 12 somatomammotropin hormone like 1 (CSHL1), elabela (ELA), activin A, soluble fms-like 13 tyrosine kinase 1 (sFlt-1), and placental growth factor (PlGF)-were quantified by ELISA in 14 blood serially collected throughout human pregnancies (20 normal subjects with 66 samples, and 15 20 PE subjects with 61 samples). Linear multivariate analysis of the targeted serological protein 16 levels was performed to estimate the normal GA. Logarithmic transformed mean-squared errors 17 of GA estimations were used to identify impending PE. Then the human gestational protein 18 patterns were compared to those in the pregnant HO-1 mice. 19Results: An elastic net (EN)-based gestational dating model was developed (R 2 = 0.76) and 20 validated (R 2 = 0.61) using the serum levels of the 6 proteins at various GAs from women with 21 normal uncomplicated pregnancies (n = 10 for training and n = 6 for validation). In pregnancies 22 complicated by PE (n = 14), the EN model was not (R 2 = -0.17) associated with GA at sampling 23 3 3 in PE. Statistically significant deviations from the normal GA EN model estimations were 24 observed in PE-associated pregnancies between GAs of 16-30 weeks (P = 0.01). The EN model 25 developed with 5 proteins (ELA excluded due to the lack of robustness of the mouse ELA essay) 26 performed similarly on normal human (R 2 = 0.68) and WT mouse (R 2 = 0.85) pregnancies. 27 Disruptions of this model were observed in both human PE-associated (human: R 2 = 0.27) and 28 mouse HO-1 Het (mouse: R 2 = 0.30) pregnancies. LEP out performed sFlt-1 and PlGF in 29 differentiating impending PE at early human and late mouse gestations. 30 Conclusions: As revealed in both human and mouse GA EN analyses, temporal serological 31 placenta-related protein patterns are tightly regulated throughout normal human pregnancies and 32 can be significantly disrupted in pathologic PE states. LEP changes earlier during gestation than 33 the well-established late GA PE biomarkers (sFlt-1 and PlGF). Our HO-1 Het mouse analysis 34 provides direct evidence of the causative action of HO-1 deficiency in LEP upregulation in a...