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Background: Maternal obesity is associated with maternal complications, including hypertensive disorders of pregnancy (HDP), and related fetal complications, such as fetal growth restriction. During pregnancy, the placenta is one of the key regulators of embryonic and fetal growth. Previous studies mainly investigated placental growth by measuring postpartum placental weight. However, the effects of obesity on aberrant placental and fetal growth might occur already in the first trimester., Objectives: Investigate associations between maternal BMI and first-trimester features of placental size and vascular development., Study Design: 870 women were included from a prospective cohort study. BMI was measured <10 weeks of gestation. Transvaginal 3D Power Doppler ultrasounds were obtained at 7, 9, and 11 weeks of gestation to measure placental volume (PV) and utero-placental vascular volume (uPVV). Associations between BMI and utero-placental (vascular) volume trajectories were assessed using mixed models, adjusted for covariates., Results: Associations were found between maternal BMI and PV (non-linear model; p = 0.022). A BMI ≥34 kg/m 2 showed decreased first-trimester PV compared to normal weight (Δ∛PV=-0.070, 95%CI -0.136 to -0.004, p=0.039). Negative associations were found between maternal BMI and uPVV (β=-0.027, 95%CI -0.041 to -0.014, p<0.001). Exclusion of women developing HDP, attenuated the association with PV (non-linear model; p=0.152), whilst the association between uPVV remained (β=-0.031, 95%CI -0.046 to -0.016, p=0.001)., Conclusion: Increased maternal BMI is associated with decreasing first-trimester uPVV, moreover, in women with a BMI ≥34 kg/m 2 the PV is decreased. These findings suggest tissue-specific changes in the placental development of women with obesity., Competing Interests: Declaration of competing interest None., (Copyright © 2025 The Authors. Published by Elsevier Ltd.. All rights reserved.)