Pregnancy is associated with a profound reduction in insulin sensitivity (IS) that contributes to gestational diabetes (GDM), but the placental factors that lower IS in pregnancy are not fully known. We conducted genome-wide RNA sequencing in 450 placenta samples from a prospective pregnancy cohort (Gen3G). Participants provided plasma samples in the 1st trimester and completed 75g oral glucose tolerance tests in the late 2nd trimester, from which IS was estimated via the Matsuda index. We used regression analyses to test differential expression of 15,232 placental genes related to maternal IS, adjusting for gestational age at delivery, fetal sex, maternal age, gravidity, BMI, and 37 surrogate variables (to account for technical variability). Among the top differentially expressed genes, lower placental expression of IGFBP1 was significantly associated with lower maternal IS (P=1.7×10-4). In 837 Gen3G women, lower IGFPB1 circulating protein was strongly associated with lower IS (r=0.50, P<0.001) and predicted GDM risk (1st trimester median levels: 23,884 pg/mL in no GDM vs 14,363 pg/mL in GDM, adjusted OR=0.55 [0.39-0.75] per quartile increase; P=1.5×10-4). The association between plasma IGFBP1 and IS was replicated in two pregnancy cohorts: SPRING (n=165 with GDM risk factors, r=0.54, P<0.001) and MOMS (n=97 matched GDM cases and controls, r=0.55, P<0.001). The association of lower plasma IGFBP1 with greater risk of GDM was replicated in SPRING (adjusted OR=0.66 per quartile increase, P=0.07) and MOMS (adjusted OR=0.45 per quartile increase, P<0.001). In SPRING, plasma IGFBP1 protein were markedly elevated in the 1st and 2nd trimesters compared to postpartum (P<0.001), consistent with placental origin. Our results suggest that the placenta expresses high levels of IGFBP1, which influences IGF-1 bioavailability and is released in maternal circulation to regulate IS in pregnancy, highlighting it as a promising early plasma biomarker of GDM risk. Disclosure M.Hivert: None. S.Karumanchi: None. C.E.Powe: Consultant; Mediflix, Inc., Other Relationship; Wolters Kluwer Health. F.White: None. C.Allard: None. K.James: None. S.Majid: None. J.C.Florez: Consultant; AstraZeneca, Novo Nordisk, Other Relationship; AstraZeneca, Merck & Co., Inc. A.Edlow: Consultant; Mirvie, Inc, Research Support; Merck & Co., Inc. L.Bouchard: None. P.Jacques: None. Funding Eunice Kennedy Shriver National Institute of Child Health and Human Development (R01HD094150)